The Weeping Angels are a group of ancient alien created by lead writer and producer of Doctor Who, Steven Moffat. Their usual form of killing involves sending their victims back through time, which creates time energy for them to feed on. When being observed by a creature, they can become "Quantum locked", a process in which they literally turn into rock and cannot move: Only when a person cannot see them are they able to move. They can also move very fast when they aren't being watched, so even if someone blinks they still have enough time to get a person. According to The Tenth Doctor, they are "as old as the universe (or very nearly), but nobody really knows where they came from". He also describes them as being the loneliest creatures in the universe.
They appear in "The Oracles of Time (Part 1)", where it is revealed that they are building up an army.