Silent

The Silents were a "memory-proof" alien species who were leaders a religious order known as the Silence. They were tall humanoids around seven foot tall, with bulbous heads and grey, mouthless faces. Their hands had four large, elongated fingers. 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 made 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 hung 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 them to remember the aliens after looking away. (DW: The Wedding of River Song)

Powers and abilities
The Silents remained hidden on Earth for hundreds of thousands of years, since the early Stone Age; they had the unique quality of being "memory-proof"; anyone who saw them would instantly forget about them the moment they looked away - even photographs and recordings. Anyone looking at the Silents would recover their memories of previous sightings, although not instantaneously.

Silents also could absorb electricity, even the static electricity from the air, into their bodies, amplify the voltage a hundredfold and fire it from their hands to kill others. These energy discharges could obliterate victims. It appeared that they had to draw the energy from an outside source; 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) 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 influenced the actions of others by issuing commands to anyone looking at them. These orders would remain in a person's mind after they forgot 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. People could be driven insane by having too many orders implanted in this manner. (DW: Day of the Moon)

The Silents were very strong, able to shatter reinforced glass merely by pressing against it. They could also smash through barricaded doors. The Silents imprisoned in Area 52 suggested they didn't need to breathe or were able to breathe in water, imprisoned as they were in tanks of what appeared to be water. (DW: The Wedding of River Song) One Silent took two direct hits from a handgun at point-blank range and survived, though it was unable to walk. (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, unnoticed by the human race. They used post-hypnotic suggestion to make humans do 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 noticed by Amy before she was distracted. Another Silent was in 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)

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. 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 "You should kill us all on sight"; it had said this to Canton Delaware, 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, and 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. (DW: Day of the Moon)

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." (DW: 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. This turned out to be a trap to get to the Doctor, and the Silents easily broke out and killed the guards. Three Silents 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, inspiring him to have Silents wear a black suit and tie.
 * 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. However, The Wedding of River Song establishes that the Silents are the leaders of the Silence.