The term 'steampunk' (variously 'steam punk') was originally a slightly sarcastic derivation from 'cyberpunk'. Cyberpunk is a flavor of science fiction that is typically set in a dystopian near-future, yet has overtones of '40s film noir. The movie Bladerunner and Neal Stephenson's book Snowcrash are quintessentially cyberpunk.
Steampunk as a descriptive term began being used in the '90s to characterize science fiction that was set in an anachronistic past, where, for instance, computers were developed in the Victorian Age. Gibson and Sterling's novel The Difference Engine is an early representative of this class of fiction. The term now embraces fiction that is not necessarily set in a variant of our past, but that has a 'feel' of Victorian sensibility about it.
When discussing the difference between science fiction and fantasy, Orson Scott Card (who writes both) has been known to remark that if it has rivets, it's science fiction. Now, of course, you don't even need the rivets, just extruded polymers. I would suggest that if it has rivets, and the rivets are highly polished brass, as are the visible gears and dials, then it's possible you are in steampunk territory. If the gentlemen all don evening clothes for dinner, you know you are. The speculative fiction of H.G. Wells and Jules Verne has been retroactively classified as steampunk by some.
Steampunk may or may not convey the noir-ish gloom of cyberpunk. Some steampunk conveys an air of Victorian innocence and optimism, but they are in the minority. Today, steampunk can even cross the boundary between fantasy and science fiction.
I would classify China Mieville's Perdido Street Station and its follow-on novels also set in the world of New Corbuzon as steampunk, although they are set on a world that humans share with other sentient species, and magic mixes with technology to create human/machine combinations. I would put this series in the steampunk category for several reasons: The people who have machine parts were altered as punishment, often for rebelling against the rulers; The ruling authority is arbitrary and unaccountable; Hydraulics, dirigibles and railroads are significant factors; Finally, Mieville's writing style evokes the sepia-tones, crowding and grime of Dickens' London.
Steampunk is a wonderful sub-genre of an already wonderful genre. If you like alternate history science fiction, and a slightly (or very) dark worldview, you will probably enjoy steampunk.