The Cybermen were originally human, but became cyborgs, mixtures of organism and machine similar to the Borg. In a process called "cyber-conversion", which was slower than Borg assimilation, they removed their organs and the flesh from their bodies, and replaced these with mechanical and cybernetic counterparts. They retained the human brain and other vestiges.
From the 23rd to 24th centuries, their design changed markedly.
The earlier type of Cyberman was immensely strong, able to break heavy doors, crush phasers with their hands and lift and throw full-grown humanoids. They were also resistant to phaser fire. However, they were vulnerable to gold dust, which interfered with their respiratory systems and disabled them for a time. (TNG - Assimilation² comic: "Issue 3")
The Cybermen were ruthless and determined, and devoted to survival by any means, particularly conquest and cybernetic conversion. They captured others and forcibly converted them into Cybermen, while destroying those who were threats to them. (TNG - Assimilation² comic: "Issue 3")
- A lower-ranking Cyber-Leader of the earlier variety of Cyberman appears in the TNG - Assimilation² comic: "Issue 3", as evidenced by its black headpiece and "handle bars". However, it has not been identified as such within a Star Trek source.
The Doctor had had several encounters with the Cybermen and knew them well.
In the late 2260s, three Cybermen of the earlier type infiltrated an abandoned relay station on Aprilia III and dominated the Federation archaeologists working there. They used a device inserted in the ear to control them: during the day, they were docile and unemotional; at nights, the devices blocked sensory input to the brain and the victims stood catatonic. On stardate 3368.5, while investigating a lack of communication from the team, Captain James T. Kirk, Mister Spock, Doctor Leonard McCoy and Lieutenant Commander Montgomery Scott, working with the Doctor, discovered the mind-control and attempted to free the researchers, before the Cybermen attacked. A battle ensued, but the Doctor knew of their weakness to gold and applied his sonic screwdriver to Kirk's communicator to generate a handful of gold dust to disable the lead Cyberman. The Cybermen were defeated and the researchers were liberated. Kirk later arranged for a garrison of Starfleet Security personnel to protect the archaeologists against further Cyberman incursions. (TNG - Assimilation² comic: "Issue 3")
The Cybermen were somehow able to alter history and bring the two universes together so they could forge an alliance with the Borg to aid them in their conquest and gain their technology. (TNG - Assimilation² comic: "Issue 4") Prior to 2368, the Cybermen entered into this alliance with the Borg. (TNG - Assimilation² comic: "Issue 1")
The Cybermen remained unknown to the Federation until, on stardate 45635.2, a joint Borg/Cyberman assault force attacked the Federation planet Delta IV and quickly overran it. (TNG - Assimilation² comic: "Issue 1") The USS Enterprise-D responded to a distress call there and ran into the fleet. Following a brief threat from the Cyber Controller, the fleets attacked and the Enterprise fled into the Paertes Nebula while the Borg/Cyberman fleet moved on to another target. (TNG - Assimilation² comics: "Issue 2", "Issue 3") The Borg–Cyberman fleet were on a course towards Earth, but stopped at Cogen V to assault that world and capture the Cogenians.
Then the Cybermen betrayed the Borg, shooting some in the back before battling in the ruins of Cogen V. The Cyberman ships also devastated the Borg fleet, and took the Cogenians for cyber-conversion. The Cyberman fleet then set course for Borg space in the Delta Quadrant. (TNG - Assimilation² comic: "Issue 4")
Within the Doctor Who universe, the origin of one kind of Cybermen was among the Mondasians. In the Star Trek universe, in the VOY short story: "Ambassador at Large", the name "Mondasian" is given to another cybernetic race of similar appearance, dwelling in the Delta Quadrant.