Silent

The Silents were a memory-proof alien species who led a religious order known as the Silence. They were tall humanoids with bulbous heads and grey, mouthless faces. Their hands had four large, elongated fingers with one bulbous central finger. They had small eyes in deep recesses in the skull. The eyes lacked sclera and were completely dark. They spoke in low, guttural voices and tended to make growling or clicking noises. They wore black suits and ties in the style of 20th century male government workers on Earth. (DW: The Impossible Astronaut/Day of the Moon) They would also hang upside down from ceilings while they slept, much like Earth bats. (DW: 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 the devices to remember the aliens after looking away. (DW: The Wedding of River Song)

Powers and abilities
The Silents were able to remain hidden on Earth for thousands of years, as they possessed the unique quality of being "memory-proof": anyone who saw them - even a photograph or recording of them - would instantly forget about them the moment they looked away. Anyone looking at the Silents would recover their memories of previous sightings, although not instantaneously.

Silents could draw electricity from anything and use it to fire an electrical discharge from their hands to kill humanoid beings. At full power, these energy discharges could utterly obliterate victims, leaving only ash. It appeared that they had to draw this energy from an outside source since when Joy was killed by a Silent, the lights in the restroom flickered and it appeared the energy was channeling into the Silent (DW: The Impossible Astronaut ). However, in the pyramid in the alternate timeline resulting from River refusing to kill the Doctor, the Silents seemed able to fire smaller, still-fatal bolts without absorbing energy from any outside source. (DW: The Wedding of River Song) When absorbing and discharging energy, a hole would appear where a mouth would be on a human's face. (DW: The Impossible Astronaut / Day of the Moon)

The Silents could influence the actions of others by issuing commands to any person who looked at them. Any order so given would remain in a person's mind even after they forgot the encounter with the Silent, and take on the form of post-hypnotic suggestion. This would also happen if the suggestion was made by someone other than a Silent to a person looking at an individual Silent or an image of one. People could be driven insane by having too many orders implanted in this manner. (DW: Day of the Moon)

History
The Silents had been present on Earth for thousands of years, since the time the wheel and fire were developed, going unnoticed by the Human race. They used their ability of post-hypnotic suggestion to guide Humans into doing their bidding in such things as going to the Moon. (DW: The Impossible Astronaut / Day of the Moon)

A single Silent spied on the Doctor, Amy, Rory, and River in Utah in 2011, briefly being noticed by Amy before Rory distracted her. Another Silent visited the White House in 1969 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". (DW: The Impossible Astronaut)

At some point, 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 control the man that had run it into thinking the orphanage was still in operation. They showed great interest in the welfare of the little girl, putting her in the life support "astronaut" suit to keep her healthy and safe - even equipping the suit with a built-in weapons system. The Silents also kidnapped Amy when she was investigating the orphanage with Canton. They took her to their underground lair and cryptically told her that she would help "bring the Silence."

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 the words "You should kill us all on sight", which it had said to Canton Delaware while he tried to help heal it after it had been shot as a jab at human mercy. This footage was subsequently viewed by 500 million Humans live, and would be seen every time anyone viewed the Apollo 11 landing footage in the future, one of the most famous and most-viewed film sequences in the history of the planet. Without even realizing it, Humanity began to exterminate the Silents as commanded. The Silents holding Amy captive tried to kill the Doctor for this. However, River killed all the Silents present before they could execute the Doctor and his friends. 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. (DW: Day of the Moon)

Two Silents witnessed Madame Kovarian kidnap River Song from the Luna University and force her in to an astronaut suit as part of Kovarian's plan to kill the Doctor, showing that at least some of them had survived the events of the moon landing. When River asked Kovarian what they were, Kovarian merely replied, "your owners." It appears they were able to undo the Doctor's damage caused in 1969, as two (apparently) human servants forced River into the astronaut suit without attacking or acknowledging the Silents, despite the aliens being in their line of vision. (DW: Closing Time) They appeared to have many human servants who served the Silence, showing that the Silents may have been able to offset the command given in the moon landing footage. (DW: The Wedding of River Song)

In the Area 52 Pyramid in an altered timeline, at least a hundred of these creatures were "captured" by Amy Pond's troops and placed in specimen tanks filled with water. However, this was revealed to be a trap to get to the Doctor, and the Silents easily broke out and killed the guards. Three Silents then attempted to kill Rory but were gunned down by Amy. (DW: The Wedding of River Song)

Behind the scenes

 * Steven Moffat has stated that he was inspired by Edvard Munch's The Scream when creating the Silence's appearance. He has also cited that the urban cult figures The Men In Black inspired him to portray the Silence as wearing a black suit and tie.
 * A single member of the species is known as a "Silent" while more than one are called "the Silents". Until clarified in Let's Kill Hitler, the Silents were erroneously presumed to be the Silence. However, The Wedding of River Song establishes that the Silents are the leaders of the Silence, and the confusion may be intentional.