The race originated from the planet Mondas, a near duplicate of Earth hidden by the Sun. After the accidental destruction of Mondas by a miscalculation of transferring (or rather, stealing) power from Earth, they invade the planet Telos and use it as a base to attempt to conquer the known universe.
Cybermen were originally a wholly organic species that began to implant more and more cybernetic parts into their bodies, including brain function. This led to their development of becoming coldly logical and calculating. Emotions were only shown when naked aggression was called for.
As the original race was limited in numbers and were continually being depleted, the Cybermen became a race of conquerors who reproduce by taking other organic beings and forcibily converting them into Cybermen in facilities called 'Tombs'. After a period of time they would spectacularly 'burst' out of the tombs as fully fledged Cybermen on an unsuspecting passer by.
The costumes worn by Cybermen acted almost as a guide to prevailing fashion at the time of transmission. Always silver in colour and included material as cloth, rubber, pvc, chest units, tubing and cricketersers sports gloves.
Early Cybermen voices sounded alien by putTING the INflecTION IN the wrong PLACES (sic). Later the BBC used special effects from its Radiophonic Workshop by adding a Electropharynx, then a vocoder to modify speech to make it sound more alien.
Their major weakness is the element gold, which fatally interferes with their life support systems.
The cybermen also used small metallic insect-like creatures called 'cybermats' as weapons of attack.
In many ways, the Cybermen can be seen as a slightly more individualistic forerunner of Star Trek's major enemy, The Borg. Similarities in speech include: Cyber-converted = Assimilation and Resistance is Useless = Resistance is Futile. The voices of the late period cybermen and the Borg are also similar.