The Cybermen were created by John Lumic, during the Second Renaissance. He intended for them to be the next step in human evolution. Under his control, the Cybermen launched an all-out assault on humanity, "upgrading" thousands of humans. After Lumic's assassination, they forged a cease-fire with the Alliance of humans, Prawns, and Vortigaunts.
Eventually, the Cybermen joined forces with the Machines, and wiped out most of humanity. They later opposed the Daleks with the aide of several other races, including the Doctor.