Steampunk is primarily about rebellion and counterculture in the Age of Steam - from about 1850 to 1910 C.E., or, more roughly, the Victorian and Edwardian eras - and Victorian scientific romance: the science fiction of Verne and Wells. It can, however, range as far as the Industrial Revolution or the American West; the key ingredients are science and rebellion against (or absence of) hegemony.
Steampunk can be strictly technology-based, as in Moore and O'Neill's The League of Extraordinary Gentlemen (although it often uses "fantastic" technology, such as Cavorite); it can be primarily fantasy-based, as in the anime Tenkuu no Escaflowne; or it can be a mixture of the two, as in the comic book series Steampunk.
Although it isn't what this community is about, the term "steampunk" has been adopted by several subcultures, including goths and punks, to represent a style of fashion and aesthetic taste.
Discussion of (hypothetical and fictional) steampunk culture, technology, settings, and similar topics. It is not a forum for discussion of the "steampunk" style of goth or punk fashion and art, although those are certainly acceptable for occasional discussion. Art, music, fiction, and other creative endeavors based on the steampunk style are also welcome here.
Where can I find more information on steampunk?
Check Wikipedia's entry. Also, check out the following creative works to get an idea of the themes and ideas present in much of steampunk:
The Difference Engine, by Bruce Sterling and William Gibson
The League of Extraordinary Gentlemen, by Alan Moore and Kevin O'Neill
The live-action Disney film 20,000 Leagues Under The Sea
The anime series Tenkuu no Escaflowne, released under the (poorly-translated) title The Vision of Escaflowne in the United States
The works of Jules Verne, H.G. Wells, and Mary Wollstonecraft Shelley
...among many others.
Although many of the posts there deal with the "steampunk" style of goth fashion, anachrotech is another community which focuses, in part, on steampunk.
steampunk was taken, and LiveJournal doesn't allow hyphens in usernames. If it had, the name of this community would have been "steampunk-l", as a nod to classic Bitnet mailing-list naming conventions.