Whenever I mention steampunk in a post I get questions about exactly the term means. To me my explanations always seem a bit bumbling.
Happily I can report that Sci-fi and fantasy giant, Tor Books has a great blog post from 2009 that explains the both the literary genre and the subculture well: Steampunk 101 by G. D. Falksen.
For reference, Falksen is the guy pictured with the fantastic leather and brass prosthetic arm.
I'd be remiss if I neglected to mention that I didn't happen across said post on my own. I first saw it referenced on Steampunk fan page on Facebook. There's that Falksen image (it's everywhere!), I don't know that he's the one who maintains the page though.