Weeping Angels
The Weeping Angels have so far been seen in three episodes of Doctor Who – Blink (Series 3 : Episode 10), The Time of Angels (Series 5 : Episode 4) and Flesh and Stone (Series 5 : Episode 5). Weeping Angels are a group of psychopathic hunters which are said to be as old as the universe. The Angels have one of the best defenses to protect themselves from harm, they cannot be observed by another being. As soon as another being looks at an Angel, the Angel turns to stone, which is almost indestructible. When they are like this they are "quantum locked" in time and space, occupying that single spot. However, they also have to be careful, because if one Angel looks at another Angel, then they will both turn to stone forever. To protect themselves from this, they cover their eyes with their hands, which gives a "weeping" impression, hence why they are called “Weeping Angels.”
The Angels have great physical strength and speed, observed in The Time of Angels and Blink respectively. They have the strength to snap human necks which make them deadly killers, which is the opposite of the quote the Tenth Doctor used in Blink - "they are the only psychopaths in the universe to kill you nicely." In Blink the Angels do not kill anyone, they simply send their victim back in time, usually to a time before they were born, where they live out the rest of their lives. The Angels then feed on the potential energy of the years their victim would have lived in the present. They can also feed on electrical energy (seen in Blink) and radioactive energy (seen in The Time of Angels).
The form of an Angel can deteriorate over time if they do not have energy to feed upon, as seen in The Time of Angels, but when they then do have energy they can restore themselves. They can communicate to other beings using the bodies of the victims they kill, again seen in The Time of Angels, where they snap the neck of a cleric soldier, remove his brain and animate his body to speak to the Doctor. It is also said “That which holds the image of an angel itself becomes an angel” which is seen in The Time of Angels, when a recording of the Angel in the Byzantium’s hold is able to control the TV screen and start to actually come out of the screen into the room. Angels can procreate by planting an image of themselves onto another beings mind by looking in their eyes. Eventually, the new Angel will erupt from the victim, killing them and producing a new Angel. This happened to Amy Pond in The Time of Angels/Flesh and Stone, though she was able to slow the gestation of the Angel by closing her eyes.