What Is Steampunk?

No seriously, what the fuck is steampunk? I believe I know what steam is, and I have a pretty good understanding of punk, but when those 2 words appear together it makes little to no sense to me. I look it up online and find pictures of people who dress like they were extras in that really shitty Will Smith movie, I think it is called Wild West or something. What do people do when they dress up like this? Are they robbing trains or stage coaches or something else equally as dumb? Can anyone explain this phenomenon to me? These wacky kids and their insane subcultures confuse me.