Probably best to explain the whole 'punk thing in general... In most contexts, it's where there's a certain overall 'theme', which would be found in stories, movies, artwork, etc. It's a 'what if' kind of thing. In the case of Steampunk, it's 'what if steam-power really took off?' and bases technological advances off of that, along with stylistic embellishments from around the time when it was around. It's a lot of brass and gears mixed with Victorian styles, essentially. However, there are other 'punks too. Dieselpunk, which envisions a world where gasoline-power rules, exclusively. Atompunk, which is heavily influenced by and elaborates on 50's and 60's nuclear power optimism and fear. Decopunk, which is one of my favorites, envisions a world that carries on the aesthetics of art deco or is set in the early 20th century with some embellishments for the sake of fiction (Sky Captain and the World of Tomorrow is a great example). And then aside from Steampunk, the other big one is Cyberpunk, which is heavily inspired by William Gibson's 'sprawl trilogy'; where the term 'cyberspace' originated. Cyberpunk tends to adapt itself with the times in certain cases, as technology develops, but it's especially stylized by the 80's and early 90's, and applies an older aesthetic to future technology. It's also tied into dystopian views of the future, where corporations govern the land, and technology has been thoroughly integrated into everything including our bodies and our fashion. With a lot of this stuff, it's just an alternate-world setting or retro-futuristic vision for fiction and fantasy.