What is Steam Punk?
Steam Punk is a retro off shoot of Cyber Punk. Many feel it started with Bruce Sterlings releasing the book "The Difference Engine". It was an alternate history where Babbage completed his Difference Engine and steam powered mechanical computers took off long before electric computers were ever considered.
Imagine outlandish technology, but where steam power had remained the primary power source instead of the combustion engine. Or just everyday devices that managed to work off of gears instead of electricity.
While most Steam Punk followers tend to gravitate toward Victorian England, remember the West was still wild at this time and it had steam engines as well. And Steam Punk can also be futuristic or current day, as modern things like nuclear and geothermal power are still just steam driven turbines.
Are there hard or fast rules about what is or what is not Steam Punk? No. Unlike most other forms of sci-fi or fandom, where people model things off of existing literature and other media, Steam Punk is very free form and you make it up as you go along. There are some common threads, though. A sense of adventure. Leaning toward the ornate. Gears are prefers over electrical devices.
There are a number of books and movies that people consider "Steam Punk", some examples:
The Difference Engine by Bruce Sterling
Mainspring by jaylake
The Amazing Screw On Head
Sky Captain and the World of Tomorrow
20,000 Leagues Under the Sea
The League of Extraordinary Gentlemen
Decor and fashion:
Jake Von Slatt is Steam Punking an RV and has made some awesome SP'ed computers.
porkshanksMakes some amazing jewelery and clothing (and weapons)
As for music, I think there is still a lot of debate as to what Steam Punk music is. The Seattle band Abney Park identifies it's self as Steam Punk. Other people refer to Rasputina as Steam Punkish. Personally, I think the Firefly theme is about as Steam Punk as it gets. It's not electronic, and it's about adventure. When I imagine Steam Punk music in my head, I think of sailors on tall ships singing songs about adventure, then I take the same style music and apply it to space travel and sci-fi.
There are a lot of subcultures that tie in with Steam Punk, ranging from the people mixing in Cthulhu lore to Flash Gordon style space travel, and even some Dr. Who-esque time travel.
Why do some people identify with Steam Punk, and in particular, Victorian England? Personally, I think we are living in a new Victorian age. In Victorian times there was rapid advancements in science and they thought they could solve any problem with a machine of some sort. Much like today, with nano technology, advances in quantum physics, medicine, and space discovery. Also, Victorian England was involved in colonization in Africa, China, etc, often resulting in unrest in those regions, much like our nation building practices. While the technology is more advanced, the attitudes of then seem to mesh well with now. It's been said that the sinking of the Titanic was what killed the Victorian Age, and brought about a lot of doubt in technology. I kind of wonder what "Titanic" we will have in our age.
Anyway, that is my rambling answer to what is Steam Punk.