Vashta Nerada, literally, "the shadows that melt the flesh", were swarming, carnivorous beings which were found on most planets.
The Vashta Nerada were microscopic beings that lived in swarms, billions strong. Individually or in small numbers, Vashta Nerada were not a threat. They were very small; the Tenth Doctor claimed that some of the dust specks visible in bright sunlight were single Vashta Nerada or small swarms, not large enough to be a threat to most life. In large numbers, they could strip a creature to its bare bones in milliseconds. (TV: Silence in the Library) In extreme cases, they could tear apart the stone bodies of silicon-based lifeforms, though it was not necessarily edible to them. (AUDIO: Night of the Vashta Nerada) Vashta Nerada swarms were sentient and two spacesuit-inhabiting swarms developed the ability to communicate with the Doctor through the spacesuits' neural relays. (TV: Forest of the Dead) They also coordinated their efforts and made the spacesuits walk, though with some difficulty; they walked slowly in a "zombie-like" fashion.
The Vashta Nerada lived in the darkness. They cast shadows when they entered the light — even stopping the reach of light from fission lamps. On their own, outside of the darkness, a Vashta Nerada swarm looked like a shadow cast by nothing. They mimicked the shadows of their prey to get close, which meant staying in the light was the only way to escape. Light also slowed them down somewhat, but if the swarms were great enough, the effects of light would be minimised and overcome quickly. If someone had an extra shadow, it was already too late. The Doctor could detect the difference between a Vashta Nerada swarm and an ordinary shadow by using his sonic screwdriver. (TV: Silence in the Library / Forest of the Dead) The Doctor believed they had no weaknesses, and the only thing to do was to run. (TV: Silence in the Library)
Besides the swarm in the Library, the Doctor indicated that the Vashta Nerada were generally harmless. Existing on over a billion planets including Earth, the Vashta Nerada inspired a fear of the darkness but were nowhere near as aggressive as they were in the Library. The Doctor stated that he'd never seen a swarm as large or as aggressive as the Library swarm. However, this unusual aggressiveness came from the fact that the books were printed with paper from the trees of the Vashta Nerada's forests and thus they saw the Library as their "forest". (TV: Silence in the Library / Forest of the Dead)
Vashta Nerada engineered on Synthesis Station possessed the ability to consume Dalekanium. A negative strain of Vashta Nerada that thrived in the light were also created, named Nerada Vashta. (AUDIO: Day of the Vashta Nerada)
Earth and "a billion other worlds", such as Funworld (AUDIO: Night of the Vashta Nerada) and the Library, had Vashta Nerada living there. The Tenth Doctor told Donna Noble that "Where there's meat, there's Vashta Nerada." (TV: Silence in the Library) They were born as microscopic spores in trees and lived their lives in forests. These spores were resilient enough to hatch even if the tree was converted into paper. (TV: Forest of the Dead)
While Vashta Nerada swarms were not common, they inspired fear of the dark in most species, since any shadow could be made of Vashta Nerada. According to the Tenth Doctor, this was why almost every species in the universe was afraid of the dark. They knew, at the back of their minds, that the Vashta Nerada were there. (TV: Silence in the Library)
On Earth, however, the Vashta Nerada were largely harmless and benign. (GAME: Shadows of the Vashta Nerada, AUDIO: Night of the Vashta Nerada) They survived by scavenging road kill, but the Tenth Doctor mentioned that some people who walked in the dark never came out. (TV: Silence in the Library)
In the 23rd century, a temporal rift resulted in hostile Vashta Nerada arriving in the underwater city Poseidon 8 on Earth, along with a giant shark-like being. The Eleventh Doctor and Amy defeated some of the Vashta Nerada by luring the diving suits they were using into a brightly-illuminated area; the Doctor later closed the rift and returned the Vashta Nerada to their original planet. (GAME: Shadows of the Vashta Nerada)
In the 50th century, one planet suffered a gigantic infestation. The Vashta Nerada usually lived in their forests, but when some of the trees were cut down and made into books to be put in the Library, some of the spores were brought along. Over a "million million" Vashta Nerada hatched from the books, (TV: Forest of the Dead) so many that when the Library's life form scanners were later used to identify their presence individually, the scanners hit their capacity of 1 trillion life forms, the maximum number they could record at any given time. (TV: Silence in the Library)
When the Vashta Nerada hit their hatching cycle, the swarms infested the entire library. In response to this, the Library's data core, CAL, teleported everyone inside its system but could not send them back out because nowhere in the library was a safe place for them to make an escape. The 4,022 people on the planet could not be evacuated and were "saved" in its computer's hard drive. Because it was impossible to ever send the people back to their physical lives, the computer created a virtual world for them to live in which hid the truth from them for their own good, (TV: Forest of the Dead) and CAL sealed the planet off in quarantine. The Library, save for its Vashta Nerada horde, was left abandoned and empty for a hundred years until an archaeological expedition which invited the Tenth Doctor and Donna Noble landed on the planet.
The Vashta Nerada slowly hunted the expedition, picking them off one by one. They first took Miss Evangelista, leaving her remains to be found by the others. Their next victim was "Proper Dave"; they first latched onto him, giving him a second shadow, and then made their way inside his spacesuit. Once Dave was devoured, the Vashta Nerada used his spacesuit to chase the other expedition members through the library. "Other Dave" was soon devoured as well, the Vashta Nearda also using his spacesuit. (TV: Silence in the Library)
When the Vashta Nerada latched onto Anita, the Doctor tried to confuse them by letting her spacesuit's visor be completely darkened so they would think they already entered her suit and would possibly leave her alone. However, because Anita's face was no longer visible, the creatures instead used this disguise to their advantage and ate her unnoticed by the others. The swarm then used her data ghost's voice in the neural relay and began speaking as though they were her, but were quickly discovered when the Doctor noticed the absence of her second shadow.
The Doctor convinced the Vashta Nerada to stop hunting the remaining expedition members and give him a chance to return the people stored in the Library's computer. While the Doctor had no way of defeating them, he knew the Library would hold information about him and had the Vashta Nerada look him up. His fearsome reputation alone was enough to make the Vashta Nerada back down for a while. They agreed to give him a day.
After Professor River Song sacrificed herself to free the people who had been "saved" by CAL, Strackman Lux organised mass teleportations, and the humans left the planet to the Vashta Nerada. (TV: Forest of the Dead)
The Doctor and River's encounter with the Vashta Nerada in the Library was mentioned in the Doctor Papers, and recounted in Doctor Who and the Silence in The Library, which was sold in a number of parallel universes. (PROSE: The Day of the Doctor)