My impression is that Steampunk is set in Victorian times, fashions, technology. Using what would have been available then, fantastical contraptions are created with appearance being an important part of the finished item. I've seen more of the costumes at cons, not read the literature, but I would assume it is an extension of the same, largely centering around good looking machines that never actually existed that do improbable things . Oh, and everything is very dark--lots of black and steel. I believe the story lines also tend to be dark.