The most succinct description of Cyberpunk I've heard: "the future if the 1980s never ended." This description applies more generally to other -punk genres:
Steampunk: the future if the 1890s never ended
Dieselpunk … if the 1940s never ended
Atompunk: … if the 1950s never ended
Curiously, we have not yet seen the emergence of a -punk genre set in the 90s/2000s.