Steampunk ranges close to Alternate History in that is is a type of alternate history, usually around the time period of Victorian England in which the civilizations then have access to more modern technology. Sometimes this technology ranges amid the fantastical but overall, it tends to lean towards science-fiction rather than fantasy. Paranormal-romance in a steampunk setting has been popular lately, which shows that steampunk can genre-hop and is by no means locked in to a tight parameter.
The Galaxy Express has a great article on steampunk and gives some examples of the genre.
Goodreads list of Best Steampunk books.
Tor.com hosts The Steampunk Workshop with articles, book examples, and such to help you understand the genre more.
Do you write or read steampunk? What's your favorite steampunk novel or author?