Unfortunately, not everyone knows what steampunk is. So before we go on, I'm going to test you using visual a visual cue.
If you're asking yourself, "Why the **** does Abraham Lincoln have a machine gun for a hand?" you probably don't understand steampunk. If you're saying "Holy **** Abe Lincoln has a machine gun for a hand! That's ****ing awesome!" then you get it.
Steampunk is an artistic and literary style that tends to take place in fictional alternate realities, typically Victorian era, or American Midwest, where high technologies are made possibility through the use of steam engines. For example, steam powered computers, steam powered tanks, steam powered mechas and steam powered arm-machine-guns are all possibilities, provided you don't ask too many questions or look TOO deeply into the physical possibilities. This results often times in a world that appears outwardly advanced, but is rife with corruption.
Steampunk was popularized in novels by preeminent writers, such as Jules Verne in his flagship masterpiece "30,000 League Under the Sea." It has inspired a number of spinoff genres, such as "Dieselpunk" and "Cyberpunk" both of which take distopic influences from Verne's work.
Movies often times use steampunk influence to make their settings more interesting, foreign or just unique. Skycaptain and the World of Tomorrow, the League of Extraordinary Gentlemen and Hellboy all have artistic styles that are heavily influenced by steampunk aesthetics.
But it is video games that benefit most from the legacy of steampunk. Recently, within this last console generation, the steampunk theme has garnered population and we have seen an influx of these steampunk themed games, and, I have to say - I'm mighty impressed with where the genre is heading.
So here's a few of my personal faves, both upcoming and released.