Fairies but have lived alongside humanity since the dawn of time, and although mankind has ascribed positive, friendly aspects to them, they are dangerous. Their exact nature is unclear. They are part myth, part spirit world and part reality jumbled together, mixed with "old moments and emotions", all moving backwards and forwards through time and seen only out of the corner of one's eye.
When they chose to be visible, they are known in two forms: a small, glowing humanoid form with butterfly-like wings and a much larger and distorted winged humanoid. They are also undetectable by technology, and can appear and disappear at will. They also have control of the elements, able to create sudden gales or rainstorms and direct them with pinpoint accuracy. They can make great storms, wild seas, turn the world to ice and have the ability to move back and forth in time.
Fairies and children are linked, and were once children, taken from various time periods stretching millennia into the past. These children are the Chosen Ones, who the fairies protect and avenge if harm comes to them, until the time that they claim the children for their own.
They are also known as Chaos Mites (Annihilation) and thought to be among the first born of the creation event. The Proemials deemed them unstable and believed to have wiped them out.
Known as Chaos Sprites, some are believed to serve the renegade Proemial Diableri to be his eyes and ears and tasked by their master to sow chaos in order, if these have fallen to the Proemial’s power or if he actually created the Sprites himself to be his own Chaos Mites remains unknown.
Source: Doctor Who / Torchwood “Small Worlds” ; Marvel Universe “Annihilation”