Silent

Members of the Church of the Silence, formerly the Papal Mainframe, the Confessional Priests (also known as Silence), were genetically engineered by the Church to allow people to confess their sins without remembering doing so, accomplished by fashioning the Priests so anyone looking at them would forget their encounter when they looked away. (TV: The Time of the Doctor) A renegade batch of Priests became obsessed with preventing the Siege of Trenzalore, and travelled back in time, influencing human history over a long period of time, and attempting to create a fixed death for the Doctor at Lake Silencio. They became known to the Doctor as Silents. (TV: The Impossible Astronaut, Day of the Moon, The Wedding of River Song)

Appearance
They were tall humanoids, around seven feet tall, with bulbous heads and grey, mouthless faces. Their hands had four large, spatulate fingers. They had small eyes in deep recesses in the skull. Their eyes lacked sclera and were completely dark. They spoke in deep, raspy voices and made growling or clicking noises. The renegade priests of the Kovarian chapter wore black suits and ties in the style of 20th century male government workers on Earth; (TV: The Impossible Astronaut/Day of the Moon) aboard the Papal Mainframe ship above Trenzalore, standard issue priests dressed wholly in black, wearing black suits with black high-collared shirts. (TV: The Time of the Doctor)

Amnesia and hypnotic ability
Silence remained hidden on Earth for hundreds of thousands of years, since the early Stone Age. They were uniquely "memory-proof"; anyone who saw them would instantly forget about them the moment they looked away — even if they saw a photograph of a Silent. Anyone looking at the Silents would recover their memories of previous sightings. Although any lifeform would forget the Silents when they weren't looking at them it was still possible to hear them, and people who knew they existed could listen for them in order to sense when they were nearby, as when River Song shot down a Silent without even looking at it. (TV: Day of the Moon)

The Silents were also impervious to at least some types of scanning devices, as River Song's bio scanner did not detect them in the underground tunnels. (TV: The Impossible Astronaut) She appeared to upgrade the software later to make sure this didn't happen again.

The Silents influenced the actions of others by issuing commands to anyone looking at them. These orders would remain in a person's mind after they had forgotten the encounter with the Silent, acting like post-hypnotic suggestions. Any instruction spoken while a subject looked at a Silent, even a picture of one, would cause this effect. "Repeated memory wipes fry your head eventually", which was the case for Dr Renfrew, whom the Silence used to keep Graystark Hall Orphanage open beyond its closing day. (TV: Day of the Moon)

The Silents employed human or humanoid operatives who wore eye drives, devices worn over the eye that artificially recorded information and fed it directly into the brain, allowing those wearing them to remember the aliens after looking away. However, they could use these to remotely kill their operatives if needed. (TV: The Wedding of River Song)

The Doctor and his companions put marks on their skin so that they knew they'd had an encounter with something dangerous, even if they couldn't remember what it was. After marking themselves so many times, the Doctor was able to figure out why they were doing it, realising that they were fighting a memory proof enemy. (TV: Day of the Moon, The Wedding of River Song)

The Doctor managed to utilise this power himself when trying to erase all records of himself from history. When it came to dealing with the Inforarium which had been selling information regarding him to the Daleks, the Cybermen and the Sontarans, the database was too sophisticated to hack, so the Doctor memory-proofed the information instead and anyone who learned anything instantly forgot it. (HOMEVID: The Inforarium)

Other powers
Silents could also absorb electricity, even the smallest amount of static electricity from the air into their bodies, increase the voltage a hundred-fold and fire it from their hands. These energy discharges could obliterate victims. When absorbing and discharging energy, a hole would appear on the Silent's face, under the skin, where a mouth would be. (TV: The Impossible Astronaut, Day of the Moon, The Wedding of River Song) During the Siege of Trenzalore multiple Silents demonstrated being able to use their electrical discharges in conjuncture with each other, to unleash one particularly large discharge. (TV: The Time of the Doctor)

The Silents were very strong, able to shatter reinforced glass by pushing on it. They could smash through barricaded doors with ease. One Silent took two direct hits from a handgun at point-blank range and survived, though it was incapacitated. However, Silents could be killed by enough gunfire; in an alternate timeline, Amy Pond killed three with a machine gun. They could also be killed by a single, direct hit from an energy weapon such as River Song's Alpha Meson blaster. (TV: Day of the Moon, The Wedding of River Song)

They could hang upside down from ceilings and lie in waiting, much like bats. (TV: The Impossible Astronaut, Day of the Moon, The Wedding of River Song)

The Silents who were hiding in Elizabethan England had the ability to render themselves invisible, thus making any who saw them immediately forget what they saw and what they were doing. These Silents were also more physically robust than other Silents, as it took at least four shots from an energy weapon like River Song's Alpha Meson blaster to kill just one of them. (GAME:The Eternity Clock)

Origins
The Silents started out in Earth's distant future as Confessional Priests — genetically engineered priests of the space church headed by Tasha Lem aboard the Papal Mainframe, whose power allowed sinners to forget their confessions. They also possessed powers of post-hypnotic suggestion. (TV: The Time of the Doctor)

At some point in time, long after the end of the Time War between the Daleks of Skaro and Time Lords of Gallifrey, the Time Lords sent a message throughout the universe through a Crack in Time on the planet Trenzalore. Banished to another dimension, the Time Lords sought the Doctor, the Time Lord who saved them at the height of the war and who was responsible for their predicament. The Time Lords' message also drew much of the universe to Trenzalore, including the Papal Mainframe and the Daleks. To verify the Doctor's identity, the Time Lords asked the Question, "Doctor who?" Due to the Truth Field over Trenzalore, when the Doctor answered correctly, the Time Lords would return to the universe through the crack, meaning Gallifrey's enemies would restart the Time War. The Doctor would not answer the question. However, were he to abandon Trenzalore, the planet would be ravaged by aliens and ultimately destroyed by the Papal Mainframe itself. Consequently, the Eleventh Doctor refused to do either, sparking the centuries-long Siege of Trenzalore. (TV: The Time of the Doctor)

The official church's position was that the Doctor must remain silent, and it planned to destroy Trenzalore should the Doctor vacate. Becoming dedicated to the Doctor's continued silence, the church became known as the Church of the Silence. Over hundreds of years, the Doctor defended Trenzalore from invasions by Daleks, Cybermen and numerous other alien species. During this time, a batch of Confessional Priests (and other associates of the church such as Madame Kovarian) became a breakaway sect, (TV: The Time of the Doctor) the religious order known as the Silence. (TV: Let's Kill Hitler) They became devoted to ensuring that silence would fall when the Question is asked. (TV: The Wedding of River Song) They travelled back in time along the Doctor's time stream to use any means necessary to prevent the Eleventh Doctor from triggering the siege in the first place. (TV: The Time of the Doctor) Their crusade became a prophecy — that silence must fall when the question is asked. (TV: The Wedding of River Song)

Their first strategy was to destroy the Doctor's TARDIS, which inadvertently caused their dilemma through the creation of Cracks in Time. (TV: The Pandorica Opens, The Big Bang) Their second was to raise a child to kill the Doctor and make his death a fixed point in time, (TV: A Good Man Goes to War, The Wedding of River Song) although this child, River Song, would eventually fall in love with and marry the Doctor, and proved to be an asset to him in conflicts with the Silence. (TV: Day of the Moon, The Wedding of River Song, The Time of the Doctor)

History on Earth
The latter plan involved them ingratiating themselves into human history over thousands of years, during which time they abandoned their priestly collars and began wearing conventional black suits and white shirts. Eventually the Confessional Priests came to be known as Silents. Having long since forgotten their original vocation as Confessional Priests, the Silents became twisted and cruel. (TV: Day of the Moon)

The Silents travelled very far back in time — thousands of years into the Earth's past — (TV: Day of the Moon) since at least the time the wheel and fire were developed. Their power allowed them to go unnoticed by the human race. (TV: Day of the Moon) The Doctor called them parasites; the Silents living on Earth were unable to develop any science or technology on their own. Instead they relied on other species. (TV: Day of the Moon) Their manipulation of humankind may all have been related to their end goal of preventing the Doctor reaching Trenzalore. For example, they used post-hypnotic suggestion to make humans do their bidding in such things as going to the Moon, seemingly to spark the development of an astronaut suit, (TV: Day of the Moon) which would be the template for suit they would in a later plan to kill the Doctor. (TV: Closing Time, The Wedding of River Song)

In 1561, Silents took notice of a time storm and the arrival of River Song and the Eleventh Doctor. They managed to separate them across time, sending the Doctor two centuries into the future. Around this time, they were experimenting with stasis field technology. They were harnessing the temporal energy to further their endevours, but River stole their generator. The Doctor and River arrived later to steal a Silence Time Capsule to break through the Daleks' Time Lock. The Silents used a piece of the Eternity Clock to change fixed points in time. The Silents were defeated by River Song and the Doctor stole the Time Capsule. (GAME: The Eternity Clock)

Silents appeared in London 1605, while Amy, Rory and the Doctor solved the mystery that involved the Rutan Host and the gunpowder plot. At least one Silent was in the Houses of Parliament when it was in orbit. It was seen by Rory Williams whilst he was fixing an EMP generator. (GAME: The Gunpowder Plot)

A Silent was in the White House in 1969 (WC: Prequel (The Impossible Astronaut)) and killed a woman named Joy in front of Amy Pond before delivering a post-hypnotic suggestion for her to "tell the Doctor what he must know, and what he must never know". (TV: The Impossible Astronaut)

Before July in 1969, a great number of Silents had taken up residence at a disused orphanage where the little girl was held. They used post-hypnotic suggestions to make the director believe the orphanage was still in operation. They showed great interest in the little girl, putting her in a life support "astronaut" suit to keep her healthy and safe — even equipping the suit with a built-in weapons system. The Silents kidnapped Amy when she was investigating the orphanage with Canton Everett Delaware III. They took her to their underground lair and cryptically told her that she would help "bring the Silence." (TV: Day of the Moon)

The Doctor inflicted a crushing defeat on the Silents when he ensured that humanity saw footage of a wounded Silent they held captive spliced into the footage of the Moon landing broadcast. In it, the Silent said "You should kill us all on sight"; it had said this to Canton, sneering at human mercy while he tried to help heal it after it had been shot. This footage was immediately viewed by half a billion humans, It would be seen every time anyone saw the Apollo 11 landing, one of the most famous and frequently viewed film sequences in the history of humanity. Without enough time to implant more post-hypnotic suggestions when humans looked at them, the remainder of the Silents on Earth in 1969 seemed doomed, although at least one did survive to 2011. (TV: Day of the Moon)

A single Silent spied on the Doctor, Amy, Rory and River in 2011 Utah. Amy noticed it before she was distracted. (TV: The Impossible Astronaut)

Two Silents witnessed Madame Kovarian kidnap River Song from the Luna University and force her into an astronaut suit as part of Kovarian's plan to kill the Doctor. When River asked Kovarian what they were, Kovarian replied, "your owners." (TV: Closing Time)

In the Area 52 Pyramid in an alternate timeline, over a hundred Silents were "captured" by Amy Pond's troops and placed in specimen tanks filled with water to prevent them from drawing electricity. (WC: Prequel (The Wedding of River Song)) This turned out to be a trap to get to the Doctor. The Silents easily broke out and killed the guards. Three Silents tried to kill Rory, but were gunned down by Amy. (TV: The Wedding of River Song)

Loyalists
Some priests, however, remained loyal to the Papal Mainframe and did not become members of the Kovarian renegade sect. The "Silents" onboard the Papal Mainframe — along with the rest of the Mainframe's crew — were converted into Dalek puppets when the Daleks invaded the Mainframe. They helped set a trap for the Doctor, and allowed three Daleks to enter the conference room where the Doctor was conversing with Tasha Lem. Later, multiple (presumably non-converted) Silents assisted the Doctor and the people of Trenzalore against the horde of the Doctor's enemies attacking the planet. Tasha Lem remarked at the sight of "old enemies" fighting side by side. They fought together for 600 years before being overrun, destroying or forcing to retreat all but the Daleks in that time. (TV: The Time of the Doctor)

Other references
When the telepathic Emily Fairfax read the mind of the Hypothetical Gentleman, a Silent was among the images she saw. (COMIC: Hypothetical Gentleman)

Inspiration
Steven Moffat has said that he was inspired by Edvard Munch's The Scream when creating the Silence's appearance. He has also cited that the urban legends of the Men in Black inspired him to have Silents wear a black suit and tie.

Additionally, the Silents' hand structure was inspired by the aye-aye, a nocturnal mammal found in Madagascar. The Brilliant Book 2012

The Brilliant Book 2012
The Brilliant Book 2012, a book that contains non-narrative based information, included numerous speculations on the Silents' activity on Earth:
 * Amy Pond theorised that the Silents created the tribal belief that cameras steal one's soul and were behind the "Maury Island Incident" in 1947.
 * River Song theorised that the Silents may also have been responsible for the Philadelphia Experiment and had encouraged wars as a form of population control.
 * Rory Williams thought the rise of 1950s alien invasion films and 1940s-60s UFO sightings might have been subconscious reactions to the Silents and that the Silents made repeated thefts of Edvard Munch's The Scream to suppress imagery of them.
 * In 1955, people in and around the Kelly-Hopkinsville farmhouse experienced lights and sound, but only a few alleged seeing aliens. After interviewing witnesses, Air Force Division man Gary F. Hodson drew what people claimed to have seen: a creature that resembled a Silent.
 * In 1961, Betty Hill and Barney Hill were abducted by Silents, who performed pregnancy tests on Betty. It became the most famous alien abduction in America.
 * A large group of Silents took refuge in a Naval shipyard in 1969 Pennsylvania.

Other matters

 * Silents appear in the Doctor Who video game The Gunpowder Plot as Easter Eggs. They have no effect on the plot and narrate various in-game descriptions of objects found in the game, often with their own particular spin on them. (GAME: The Gunpowder Plot)
 * A single member of the species is known as a "Silent" while more than one are called "Silents". Until clarified in Let's Kill Hitler, the Silents were presumed to be synonymous with the Silence. The Wedding of River Song establishes that the Silents are the leaders of the Silence. Whether "Silents" is indeed the name of the species, or simply a name applied to them by their followers is, to date, unknown.
 * Their reliance on other races for ideas is similar to that of the Eternals, who also relied on humans (among others) for ideas.