Steampunk is a sub-genre of Science Fiction. It's usually set in an alternate 19th century and features a lot of steam-powered machines and mechanics. It's a lot of out-of-time and place technology.
Fashion, art, and literature can all have elements of steampunk in them (like my outfit above!). One of my favorite teen steampunk series is Leviathan by Scott Westerfeld.
Leviathan is set in an alternate World War I era (so that means around 1912-1914). The Germans are using mechanized weapons while Great Britain has fabricated creatures genetically. The two sides greatly oppose the other. The heroes of the series are Aleksander, son of Archduke Ferdinand, and Deryn, a Scottish girl who poses as a boy so that she can join the British Air Service.
I don't want to give too much of the plot away- but it is an exciting and fun read. Be warned, it ends in a cliffhanger so you'll probably want to pick up Behemoth too. The series ends with Goliath.