Cybernetics describe any part of the human body that has been replaced with a machine copy or augmentation. Cybernetics wire human nerve endings to computer systems that control and actuate the device, however very few of these cybernetic parts have any sense of touch or feeling in them. The most common cybernetics can not be passed off as anything but mechanical parts, however the more advanced systems have synthetic skin grafted or sprayed onto them to pass for human flesh at first glance. Only the most expensive can pass for human under close inspection though.
Cybernetics were originaly introduced by the UAC as a way for injured soldiers to continue to lead normal lives or even continue to serve in the 3rd Great War. Much more advanced than any previous form of prosthetic, cybernetics could also increase a subject's strength and endurance. After all, a runner with mechanical legs tires less quickly and someone with a pair of cybernetic arms and supporting spinal structure could lift many times more than humanly possible. Modern medicine has allowed for some relatively painless cybernetic procedures, however most of the procedure must be done with the patient awake. The surgery can be unnerving to witnesses and the patient, the body shakes and jolts from electrical stimulus as the computer learns to read electrical impulses from the brain. The body, after all, has to get used to giving orders to a non-organic machine.
Cybernetics have since, after the war, filtered down into the lower classes as a means of extreme rebellion and thrill-seeking. These people have healthy tissue removed and replaced with cybernetic augmentation for the durrability and power such an aparatus brings to them. It is also seen as the ultimate kind of rebellion against the establishment and against one's own body: think of the most extreme body piercing in the world, now imagine that was somebody's eye.
This also lends itself to a certain social stigma concerning all cybernetics, prosthetic or augmentation. Biomechs, as they are refered to, suffer exclusion and persecution from those who believe there is something inhuman with the merging of man and machine.. They posses powers and abilities most humans do not have from infra-red vision, the ability to lift great weight or hidden weapons in their hands. Also the fact it is often impossible to tell apart those who needed the replacement and those punks who volunteered to have their bodies chopped does little to add to the reputation of the 'mechs. They are often lumped in together as social misfits at best, or a walking danger to society at worst and any crime involving a cyberneticly altered person is highly televised and reported on, further fueling the public's distrust.