The Cybermen are a race of augmented humanoids who have replaced bodyparts with cybernetic technology, and have also removed emotion. The hierarchy can include a Cyber Lieutenant, Cyber Leader and Cyber Controller.
In the normal universe, the Cybermen originated from Earth's twin planet Mondas that broke out of the solar system centuries ago but returned to Earth in 1986 (The Tenth Planet). Another known colony is Telos, where the race was once thought to have died out (The Tomb of the Cybermen).
An alternative origin of the Cybermen was encountered by the Doctor in a parallel universe, where here they were created to become a new race to follow the crippled entrepreneur John Lumic. Rogue members of this version have also travelled to the normal universe.