Steel giants with the strength of ten men, the Cybermen were originally human beings, before their weak mortal flesh was replaced with metal and plastic. In the process they lost their compassion, as well as all their other emotions.
Originally from Mondas, Earth’s twin planet, the Cybermen were thought to have died out in our universe. But when the Doctor visited a parallel world, he found they’d been created again there, by the insane genius John Lumic. These Cybermen sought to "delete" all who stand in their way.
The Cybermen voyaged across the Void to invade Earth, in the past as well as the present, building apelike Cybershade creatures to serve them and launching their ultimate weapon, the giant Cyberking. The last time the Doctor encountered them en masse, he hurled them back into the Void.
The Eleventh Doctor discovered a Cybership that had crashed on Earth hundreds of years ago, and over time the Sanderson & Grainger department store was built over the top of it. When the corroded Cybermen awoke, they began kidnapping humans and using their body parts to replenish themselves. They were destroyed when Craig Owens overloaded their emotional inhibitors as they tried to convert him into a new Cyber Controller.