Daleks were mutated descendents of the Kaleds of the planet Skaro. They fought the Time Lords in the Last Great Time War, resulting in the almost total destruction of both races. They regarded the the Doctor as their greatest single enemy and were hated and feared throughout time and space.
Externally, Daleks resembled human-sized peppershakers, with a single mechanical eyestalk in a rotating dome, a gunstick and a manipulator arm. The casings were made of a material that was also called dalekanium. The lower shell was covered with many hemispherical protrusions, seen as a self-destruct system.
The Dalek's eyepiece was its most vulnerable spot, and impairing its vision often led to its main weapon being fired indiscriminately. The Dalek casing also functioned as a fully-sealed environment suit, allowing travel through the vacuum of space or underwater without the need for additional life-support equipment.