The Cybermen used to be human. They began experimenting with cybernetic parts in order to survive, over time they became more cybernetic than human. They also had all of their emotions removed, they can no longer remember what it’s like to be human. They believe having no emotions is a better way of life.
In Rise of the Cybermen a mass upgrading Cyberman factory was shown.
On Telos, a smaller upgrade station was shown inside Cyber Control.
John Lumic used the ear pods created by Cybus Industries to control people and force them into being upgraded.
Humans that have been given an upgrade.
TobermanIn The Tomb of the Cybermen, one of the archaeologist team members Toberman was partially upgraded into cyber form.
SlavesOn Telos the Cybermen were using slaves partially converted into Cyber form.
LyttonIn Attack of the Cybermen, Lytton was captured by the Cybermen and had started to be upgraded. The Doctor was unable to help him.
Miss HartiganMiss Hartigan was upgraded to the Cyberking in The Next Doctor.
Craig OwensCraig Owens was partially converted into cyber form but the full conversion failed because the emotional sub systems rebooted.
WebleyWebley was the owner of Hedgewick’s World of Wonders.
The DoctorThe Cyberman consciousness known as the Cyber-Planner tries to take over the Doctors brain.
Inside a Cyberman
Some of the cybernetic parts that make up a Cyberman.
Life Support UnitIn The Tomb of the Cybermen a Cybermans life support unit was damaged.
Distress SignalInside every Cyberman is an in-built distress signal.
Emotional InhibitorEvery Cyberman has an emotional inhibitor, blocking out all emotions. This was originally put in place to stop the physical and emotional trauma caused by the conversion process.