The Cybermen were originally human beings, but gradually they replaced their weak mortal flesh with metal and plastic. In the process they lost their compassion, along with all 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, and could electrocute with their touch.
The Eleventh Doctor discovered a subterranean spaceship that had crashed to Earth centuries earlier. It contained Cybermen whose ultimate aim was to convert the planet to 'Cyberform'. They were defeated by Craig Owens when his paternal feelings for Alfie delivered a fatal emotional influx. Or to put it another way, he blew them up with love.