Cybermen are one of the more famous enemies of The Doctor in the Doctor Who franchise. Cybermen originate from Earth's twin Planet called Mondas, sometimes referred to as the Tenth Planet. The inhabitants decided to explore the Universe and managed to work out how to move the planet. Throughout their journeys, their bodies became weak and ill-suited to travel, so body parts were replaced to compensate until 90% of each person was robotic. Mondas required so much power to operate that it would absorb energy from any planet it came across. In a desperate search for energy, it returned to our solar system to absorb Earth's energy, but it absorbed too much and was destroyed. The surviving Cybermen began a search for a new world. The planet they came to was Telos which they found to be suitable relegating the Telosians into second class inhabitants. Like the Borg in the Star Trek series, they can only increase in numbers by converting people. The aliens are unisexual, females are converted into Cybermen. However in the Torchwood series, the episode Cyberwoman, you see a woman in the middle of being converted into a cyberwoman.
Cybermen have become one of the Doctors most frequently occurring enemies along with the Daleks and the Master. In the new series from 2005 onwards, some Cybermen do not come from Mondas but from an alternate universe. In the Alternate Universe, the Cybermen are the creation of John Lumic, head of Cybus Corporation. Cybus' intentions with the Cybermen are just as evil as the Cybermen from our universe. The Cybus Cybermen have managed to open a portal between both universes and have travelled between them.
Cybermen are controlled by a Cyber Controller. Cybermen don't tend to have guns nowadays, they incapacitate people with their hands. Cybermen fire bolts from their wrists however in the episode where the Daleks