A terrifying creature, and deadliest of assasins; The weeping Angels are very old and without mercy. They are beautiful statues...when you can see them.
"Don't blink. Don't even blink. Blink and you're dead. Don't look away, don't turn your head, and don't blink. . . . Good luck" - The Doctor
They are incredibly fast, but when you can see them the turn to stone. They cannot move, it's a fact of their biology. That's why they cover their eyes; They're not crying, they can't risk looking at eachother.
If one touches you, you will instantly find yourself many years in the past. This is how they feed. They live off of the potential energy of all the days you might have had.
Also, if you are unluckey enough to find yourself in the prescence of one, keep watching it but do NOT look it in the eye. "Anything that holds the image of an angel becomes and angel." This includes televisions AND people (eyes).