A cyborg (from cybernetic organism) is a hybrid mechanical-biological being. As such it is different from a robot, which is a fully mechanical being. Cyborgs are also to be distinguished from humanoid pseudo-animals, which genetically engineered but fully biological beings.
There two types of cyborgs: modified humans and robots with biological components. Using a very broad definition, cyborgs of the first type already exist and are actually quite common: people with pacemakers or bionic limbs and there even experiments with brain implants.
Second-type cyborgs are a little bit more futuristic, but even they exist. For instance professor Bill Dito has used leech neurons to create “a wetware computer”. Such wetware computer could be used to control a mechanical body.
The fact that cyborgs are far from a just a theoretical possibility, they are a fascinating topic for authors of mundane science fiction as cyborgs pose interesting questions. When should cyborgs considered and treated as humans?
At the moment I am planning two cyborg stories. One will feature a human being which becomes a cyborg. The other one will feature cyborgs of the second type.