Cyberpunk is a sub-genre (branch) of Science-Fiction distinguished from all other science-fiction by its sense of humankind's exorbitant reliance on computers, to the extent that our daily environments and even sometimes our bodies have been overrun or replaced by them. Its defining characteristic is advanced digital technology, such as artificial intelligence, computer viruses, holograms, virtual reality, and robots varying in form from androids (human-shaped ones) to nanobots (microscopic ones).
In essence, it is a romanticized revival of the dystopic atmosphere first imagined by George Orwell in his novel "1984" and by Bruce Bethke in his short-story "Cyberpunk", from which the term originates.