Steampunk is kinda like cyberpunk, only steamier, but not steamy as in romance; more like steamy as in steam engines, but not steam engines like in historical fiction; more like steam engines as in science fiction, which doesn't have many steam engines, which is why there's steampunk. Got it? Good...
Like a lot of genres and sub-genres steampunk isn't exactly new, (it's arguable how long it's been around) but there are always new entries and new people (like you!) to discover the old ones. Scott Westerfeld's Leviathan, Kenneth Oppel's Airborn series and of course Fullmetal Alchemist are some of the more recent and popular examples of Steampunk which combines historical settings (usually Victorian) with technology beyond what was available at the time. For example, a steam powered airship or... even more steam powered airships! (there's a lot of steam powered airships). Sometimes the plot is set in an alternate world, or sometimes just an alternate reality, but in either case it's always fun to see the ways in which society is reinvented through this clanky, gear driven, yet futuristic technology.
The roots of this genre go very deep, as such classics as H.G Well's War of the Worlds and my personal favourite, Jules Verne's 20,000 Leagues Under the Sea (giant squid vs. submarine what's not to love?) can be regarded as inspiration for Steampunk as they introduced extremely advanced technology into a Victorian setting while actually being published during Victorian times! (1898 and 1867).
Will similar genres emerge about our own time? What would we call it? Gasolinepunk? ipunk? e-punk? Let's hope so... or not... but check out one of these titles in what is definitely a quirky, imaginative and very fun genre.