The Silence

The Silence (or Silents) were a mysterious species that forced the Doctor's TARDIS to explode on the date of June 26th 2010, and thus created cracks in time that would erase various things throughout the universe, and eventually the universe itself. They acted as parasites, using post-hypnotic suggestion to have other races construct things for them.

Biology
The Silence were tall humanoids with bulbous heads and grey, mouthless faces. Their hands possessed large, elongated fingers with a 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. (DW: The Impossible Astronaut) The Silence would also hang upside down from ceilings while they slept, much like Earth bats. (DW: Day of the Moon)

Powers and abilities
The Silence were able to remain hidden on Earth for thousands of years, as 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 Silence would recover their memories of previous sightings, although not instantaneously.

The Silence could use an energy discharge from their hands to kill and obliterate humanoid beings, leaving only a little 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 were flickering on and off. When absorbing energy and discharging it, a hole appeared where a mouth would have been on a human's face. (DW: The Impossible Astronaut / Day of the Moon)

The Silence could influence the actions of other species by ordering any person who looked at them. Any command would remain in a person's mind and take the form of post-hypnotic suggestion. People could be driven insane by having too many orders implanted by the Silence. (DW: Day of the Moon)

History
The Silence had apparently 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 unconsciously doing their bidding in such things as going to the moon. (DW: The Impossible Astronaut / Day of the Moon)

The Silence first took control of the Doctor's TARDIS in 102 A.D. They forced it to land outside of Amy Pond's house on June 26th 2010, and it exploded shortly afterward, trapping River Song inside it within a time loop activated by the TARDIS safety protocols until the Doctor rescued her using a Vortex Manipulator..

Although the entity that caused the Doctor's TARDIS to explode was never visible or named, it was heard speaking before the TARDIS began exploding, repeating: "Silence will fall!" (DW: The Pandorica Opens / The Big Bang)

In 1580 Venice, Rosanna Calvierri said that through some of the cracks, the Saturnynians saw only silence and the end of all things. This caused them to flee their homeworld to 1580 Earth through a different crack that acted like a wormhole. (DW: The Vampires of Venice)

A Silent spied on the Doctor, Amy, Rory, and River in Utah, 2011, briefly being noticed by Amy before Rory distracted her. Another SIlent visited the White House in 1969 and killed 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) The Silence had taken up residence at a disused orphanage where the little girl had come from and used post-hypnotic suggestion to control the man that had run it previously into thinking the orphanage was still in operation. The Silence later kidnapped Amy for unknown purposes when she was investigating the orphanage with Canton and cryptically told her that she would help "bring the silence." In addition, they showed such interest in the welfare of the little girl as they put her in the life support astronaut suit to keep her healthy. (DW: Day of the Moon)

The Doctor inflicted a crushing defeat on the Silence 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". This footage was viewed by half-a-billion humans at the time, and would be invariably viewed by their descendants. This sealed the fate of the Silence; humanity began to exterminate Silence as commanded. The group of Silence holding Amy captive tried to kill the Doctor for causing them to lose control of their empire. However, River killed all the Silence that were present. Without enough time to implant more post-hypnotic suggestions when humans look at them, the remaining Silence seemed doomed. (DW: Day of the Moon)

However, it appears that at least one Silent had survived the decades of massacre initiated by the Doctor and was present at the death of the Eleventh Doctor in Utah in 2011 when an unknown person in the Silence's astronaut suit repeatedly shot the Doctor to prevent him from regenerating into his twelfth incarnation. (DW: The Impossible Astronaut)

Behind the scenes

 * Moffat states that he was inspired by Edvard Munch's The Scream when creating the Silence's appearance.
 * Part of the lyrics of the Songs of Captivity and Freedom translate to "While we are silent, we are screaming." Whether this was intentional or not is unknown.