Although there is a lot of room for variation within it, Steampunk is basically a genre which emphasizes the technology of the industrial revolution and offshoots of it- either things that could have been discovered in that timeframe but weren't for some reason, or which follow scientific theories which were believed at the time but were later disproven. The best existing resources have already been mentioned, but if you want more, let me know and I will dig out some of the things I found while doing research.
Limper-I disagree that electricity is mutually exclusive with steampunk. As long as it is just a strange, experimental technology, rather than the significant part of life that it became in the 20th century, it fits right in with the atmosphere of innovation that the genre tries to foster.
Now-on to the shameless self-promotion: I am co-writing a d20 steampunk supplement, Steam-trains & Sorcery. The focus is on incorporating steampunk elements such as technological environments and technology-using characters into any campaign, but the material is valuable for any campaign within the genre, including a more traditional pseudo-historical, magicless setting. If you're interested, it will probably released as a PDF sometime this summer, with a print version following later if the PDF raises enough money.