Weeping Angels

With the appearance of marble statues, the Weeping Angels are an ancient race of humanoids who are known primarily for their main defense mechanism: to go into quantum lock and turn to stone, assuming the apperance of stone statues, if observed.

Biology
Little is known of the actual Biological make-up of such a creature primarily because it is theoretically impossible to examine them: examination means we could see them and to see them would start their ability and turn to stone. However, they are also quite fast when not observed, with the apperance of them being statues helping them survive and look harmless.

Even in their unobserved form (i.e. when moving), they continue to have the apperance of a marble-laden creature. Their senses (vision, touch, hearing) are working normally. They cannot speak human languages unless they have a human vocal cord (which they obtain by murdering humans.) While they eat things like energy, electricity, and radiation, they are nonetheless shown to have sharp, pointed teeth.

Their name comes from an adaptation that they have evolved. They look "weeping" as they are frequently seen covering their eyes, so as to prevent two angels from looking and freezing each other for eternity. It is thus notable that to put a mirror in front of an angel is tantamount to imprisoning it forever. Due to the widespread distribution of humans across the Doctor Who universe, their defense mechanism has worked quite well.

The angels not just break the laws of earth biology, but also that of physics. While they can kill (and sometimes dismember) humans with chilling skill, their main attack is to transport a victim into the past, feeding on the "potential energy" of the lives the people could have led, and effectively "killing" the person in the present time.

They have the same height and dimensions as that of a human, making them easily assume the apperance of a human marble statue and thus appear totally harmless.

Reproduction
According to the Book of the Weeping Angels, (a book on them that does not have any images of them,) angels have a unique, parasitic, and terrifying way of reproduction: image-based reproduction: "the image of an Angel is itself an Angel."

Examples of this shown in the episode The Time of Angels, where an angel's video image, captured by a security camera and shown on a television, gains awareness on its own. Another is when Amy Pond stares at an Angel's eyes, "trapping" an angel's image in her eyes. An angel could have emerged from her and killed her, yet this "infant" angel was killed by shutting off the visual centre for a relatively long time.

However, angels are not immortal, as like the stone statues they mimic so well, they are worn apart by the elements, a process that is their equivalent of aging.

History
No one knows how the angels came to be, however, their powers and abilities to bend the laws of physics suggests that they are a very ancient race.

They have been known to attack human colony worlds and committ wholesale chrono-murder of civilizations, such as the annihilation of the Aplans in the 47th century.