The Silence

The Silence 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.

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 spoke in low, guttural voices and tended to make growling or clicking noises. (DW: The Impossible Astronaut)

Powers and abilities
The Silence were able to remain hidden on Earth as anyone who saw them would instantly forget the moment they looked away. Someone looking at the Silence would recover their memories of previous sightings while they looked at them (though not instantaneously).

In addition, the Silence seemed to be able to physically move from place to place very quickly, possibly instantaneously.

The Silence could also use an energy discharge from their hands to kill and obliterate someone, leaving only ashes behind. It appeared that they have to draw this energy from an outside force though, as when performing this act the energy from the lights flew into it. When absorbing the energy and discharging it, a hole appeared where a mouth should have been. (DW: The Impossible Astronaut)

History
The Silence first took control of the TARDIS in 102 A.D. They forced it to land outside of Amy Pond's house on June 26th 2010, and it began to explode shortly afterward, trapping River Song inside of it within a time loop until the Doctor saved her. Although the entity behind the voice was never visible or named, it was heard to be speaking before the TARDIS exploded, saying: "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 Earth. (DW: The Vampires of Venice)

The Silence were apparently present on Earth for hundreds of years, going unnoticed by the human race. One of them spied on the Doctor, Amy, Rory, and River in Utah, 2011. Another one visited the White House in 1969 and killed Joy in front of Amy Pond. (DW: The Impossible Astronaut)

Behind the scenes

 * Marnix Van Den Broeke portrayed the Silence in The Impossible Astronaut.
 * Monsters created by Steven Moffat are usually designed to make something mundane and everyday into something terrifying (stone statues, gas masks, shadows). In one sense, The Silence stray from this pattern because they are distinctly alien-looking, with large, bulbous heads and elongated fingers. In other ways, The Silence stay with the pattern of turning the mundane into the terrifying in that they wear human-style suits and ties, and are said to be responsible for that "something in the corner of your eye, a creaking in your house, or voices through a wall" (DW: Day of the Moon ) which are everyday occurrences that humans experience.
 * Moffat states that he was inspired by Edvard Munch's The Scream when creating the Silence's appearance.