Weeping Angels are one of the Doctor's oldest enemies. No one knows what they are or where they come from.
If you see them, don't Blink, Turn Away or Look Away as then they move closer. They will break your neck or send you back in time with and feed off the potential life you could have had.
It is impossible for a living creature to know what they look like; if someone sees one, it appears to be a statue angel. They move very fast, so fast that they can cross a room in the blink of an eye, hence the catch phrase: Don't Blink.
When an angel has not fed off energy for a very long time can lose its wings and begin to crack, however, it is still extremely dangerous.
No pictures exist of a Weeping Angel, because if something holds an image of an Angel for more than three seconds, it will become an Angel.
They were encountered by both the Tenth Doctor with Martha Jones and the Eleventh Doctor with Amelia Pond.