Time field

A Time Field was a field of time energy that spilled from mysterious cracks in the fabric of time and space caused by the TARDIS exploding on 26th June 2010, after coming under the control of an unknown entity, whose voice was only heard repeating the line: "Silence will fall."

Cracks
The cracks were described as "two parts of space and time that should never have touched". They were present in the very fabric of spacetime - a crack that appeared to be part of a wall would still be there if the wall were removed. 

According to Rosanna Calvierri, the cracks ranged in size from tiny to "big as the sky", and some connected to other worlds, while others only to "silence, and the end of all things". 

Some cracks acted like wormholes, and could be opened to allow a passage between the places on either side. One device known to be able to open such a crack was a sonic screwdriver. (DW: The Eleventh Hour) Rosanna Calvierri and her family traveled through such a crack to Earth, as did Prisoner Zero.

Regardless of the size, and what was on the other side, all cracks appeared to be of the same shape and orientation. When they erased people and events, the consequences would still remain, as seen with the Byzantium remaining crashed when the Weeping Angels, who casued it, were erased, and Amelia Pond still existing when her parents were erased from history. (DW: Flesh and Stone, The Big Bang)

Origin
The cracks in time originated from the destruction of the TARDIS when it came under the control of an unknown disembodied external force, which forced it to materialise outside of Amy Pond's house on the date June 26th 2010. River Song tried to prevent the explosion, but lost complete control of the TARDIS, leaving it to explode, cracking various points in time and space, some times and places of which the Tenth and Eleventh Doctors had been involved in. (DW: The Pandorica Opens)

Earlier in the the Eleventh Doctor's own personal timeline, when sealing the crack in the hull of the Byzantium, he traced the origin of the crack to a temporal explosion on 26th June, 2010. This day was to be the wedding of Rory and Amy, but after the former's death and removal from existence by a crack, the wedding plans no longer existed.

When a crack appeared in 2020, underneath a small village in Wales, the Doctor was able to put his hand in it, and retrieved a piece of shrapnel from the explosion. This piece turned out to be a part of his TARDIS, which had been broken off and burned. 

Time Field
Some cracks released energy of pure time that could wipe individuals out of time itself and remove events from history, though time-travellers such as the Doctor still had the ability to remember them, at least so long as the removed event or person did not relate to the person's direct past. The Doctor guessed that such cracks had erased events such as the CyberKing walking over London in the Victorian era and the Dalek invasion of 2009, one of Earth's most publicly-visible invasions, explaining why Amy Pond did not know about those events.

The Doctor stated that the only way to close such a crack was for it to consume a complicated space-time event, such as himself or a large group of Weeping Angels (all of them together being equivalent to himself). River Song volunteered to let herself be consumed, but the Doctor laughed at her idea, and stated that she wasn't even as complicated as one Angel. 

This time field was visible as a glow of bright white light, which sometimes extended tendrils out from the crack toward nearby people and objects.

Chronons, the elementary particles of time energy, were responsible for erasing objects from the present that no longer had a past. Whether this is related to the time energy seeping from a crack erasing individuals out of time is unclear.

Notable cracks
In Easter of 1996, a crack in Amy Pond's bedroom connected to an Atraxi prison. Cracks also appeared on the side of Starship UK in the 33rd century and in the Cabinet War Room in 1941. 

Another crack later opened to an incredibly large size aboard the Byzantium spaceship in the 51st century. It erased Crispin, Marco, Pedro, Phillip and many Weeping Angels from existence and subsequently closed. The Weeping Angels had feared it, and attempted to escape, only to be dropped into it after they absorbed all of the Byzantium's energy, disabling artificial gravity. The Angels, combined, were a sufficiently complex space-time event to seal it for a while. 

The crack was mentioned again in Venice, 1580, when Rosanna Calvierri claimed that she and the other vampires fled their planet because of the "silence" that they saw from some of the cracks. They passed through one of the cracks that led to "an ocean like ours", which turned out to be the Earth, after which the crack closed behind them. Just before the Doctor left, he noticed that silence fell over Venice. 

There was a crack beneath the Earth in 2020 that the Doctor, Amy and Rory found while leaving the Silurian base. Attempting to investigate the crack, the Doctor reached inside, searching for “shrapnel” from the original explosion, only to find a piece of the TARDIS door, burned and destroyed. Before leaving, Rory was shot by the Silurian, Restac, and came in contact with the light from the crack before the Doctor and Amy could rescue him. Despite Amy's efforts, the Doctor was forced to leave Rory behind, the crack erasing Rory from existence. Although the Doctor still remembers him, proven by the Doctor mentioning him in later episodes (DW: Vincent and the Doctor) and the engagement ring which he bought Amy is still visible in the TARDIS, Amy has lost all recollection of Rory due to the crack directly affecting her past. 

There was a crack in Craig Owens's flat in 2010 in the kitchen, next to the fridge. The mysterious Time Field energy began to emit from it, and the crack began to expand. (DW:The Lodger)

Another crack appeared in the TARDIS whilst it was parked next to Amy's house on 26th June 2010. The TARDIS monitor read "LOCATION: Earth, DATE: 26/6/2010", until the glass cracked into the shape of the cracks. This was followed by a sinister voice rasping "Silence will fall". (DW: The Pandorica Opens)

Chronological appearances

 * 1941 - A crack appeared on the wall of Winston Churchill's Cabinet War Room when the TARDIS dematerialises.
 * 1996 - A crack appeared on young Amy Pond's bedroom wall, connecting it to an Atraxi prison. This was when the Doctor first noticed them appearing. The crack reappeared and was closing. (DW: The Big Bang)
 * 2010 - A crack appeared on a screen in the TARDIS before the Doctor turned it off.
 * 2010 - A crack appeared by the side of the fridge at the house of Craig Owens, where the Doctor was staying and was growing when Amy Pond opened her engagement ring
 * 2010 - A crack appeared on a screen in the TARDIS, followed by an unknown voice reapeatedly saying "Silence will fall" soon after River Song left the TARDIS. (DW: The Pandorica Opens)
 * 2010 - A crack appeared on the TARDIS screen. (DW: The Big Bang)
 * 2010 - A crack appeared on Aickman Road as Time was rewinding. (DW: The Big Bang)
 * 2020 - A crack appeared in the rock of a Silurian base. After Rory Williams was shot, it consumed him, and so was erased from time. The consequence of this was that Amy Pond forgot about him, with only the Doctor remembering him.
 * 3295 - A crack appeared on the side of the Starship UK, just after the Doctor had left.
 * 51st century - Sometime in the 51st century, a crack appeared in the forest inside the Byzantium and consumed the Weeping Angels.

Behind the scenes

 * The time crack was inspired by a crack Steven Moffat saw in his son's bedroom, which resembled a smiling mouth.
 * The Cracks arc is, like the previous series (excluding the Bad Wolf meme), foreshadowed in a previous series, in this case in, which aired in the Christmas of 2008; Russell T Davies did allow Steven Moffat to place hints to the Series 5 story arc as far back as the Series 4 finale.
 * The cracks have caused much debate amongst fans, with many "supposed cracks" that are just items of scenery (e.g., small cracks in the walls under the Earth in Cold Blood and clouds and lighting giving the impression of a crack in the sky in The Vampires of Venice, although Doctor Who Adventures issue 169 pointed it out, saying "Did you spot this sneaky crack in the sky?"). It has also been conjectured that the TARDIS keyhole, shown to lead into the time vortex in the closing scene of The Vampires of Venice, is a miniature crack turned 90 degrees to the left.
 * In recent discovery, NASA found what appears to be a crack shaped similar to the one in Doctor Who found in the middle of the Milky Way.
 * In the Doctor Who Confidential episode for The Big Bang, Steven Moffat reveals that the Silence (and presumably the voice heard in the TARDIS during The Pandorica Opens) will be explained more in series 6.