My apologies since this is LONG overdue. I have found that though steampunk has experienced a recent explosion in popularity, there are still plenty of people who are not familiar with what steampunk is, though they may have heard the term before.
There is a bit of difficulty in explaining steampunk as a genre, since it really is quite adaptable and flexible, and extends well beyond a genre of fiction to an aesthetic in clothing, art, and everyday items. As a result this is just a brief overview of steampunk as I see it.
Steampunk is a subgenre of speculative science fiction, that usually takes place during the Victorian time period, or in a world where Victorian aesthetics and ideologies are dominant. However there is the very important addition of technological advances- often steam driven- that did not necessarily exist during the time period and may be far more advanced than even our current technologies, but are always in keeping with the aesthetics of the time period. The technology is often used to try and better the lives of the people and erase the inequalities of society that were so dominant during that time period.
Steampunk, at its earliest, was influenced by writers like H. G. Wells, Jules Verne, and Mary Shelley, since their works included many fictional technological inventions and prominent elements of science fiction. Though steampunk is often set in Victorian England, you can easily find many stories set in other countries and areas, some settings altogether fictional.
Because there is so much more to steampunk than what I've covered, I encourage you to check out Wikipedia and Tor for more information. Also check out the great postings Tor put up for steampunk month in addition to a great source of all things steampunk via NaNo WriMo.
