The term Steampunk was coined in the 1980s by KW Jeter in reference to the works of Tim Powers and James Blaylock, as well as his own writing. Early works were variants on Cyberpunk but with steam powered machines taking the place of cybernetics and advanced computers.
Often dark tales of a dystopian past (just as Cyberpunk stories are tales of a dystopian future), Steampunk has now grown to include the utopian ideals of the Victorian age itself.
Steampunk tends towards the real (though early) or the theoretically possible developments of the Victorian age. In Steampunk, steam power (and, sometimes, alien influence) has lead to the invention of technologies far in advance of the industrial revolution.
Steampunk has also grown to encompass many different categories and genres; adventure, romance, gothic horror, etc. From its origins in prose (US), it has become a staple of graphic novels (US), films (US) and role-playing games (US).