So here is my answer.
Steampunk is a form of science fiction. It's an artistic expression of tomorrow's technology with the flare of yesterday's design. Plenty of brass and leather where other genres of Sci-Fi have aluminum and plastic. It's not really a new phenomenon as it's been around since the 1980's or so, but it's definably a design theme that is gaining in popularity all over the place right now. I guess you could call it (to steal a phrase from my friend Mary Paul) "Steampunk is today what Goth was 10 years ago."
For a little more specific info about the origins check out wikipedia:
Also, the best answer I've found so far was on MTV's site:
The short film Steam Driven Movie http://www.steamdrivenmovie.com/ is totally going to be a beautiful example of steampunk once it's all finished up. Some brilliant direction choices, amazing costumes and great set dressing is all going to work together to create an amazing visual experience.
Dr Grymm is one of the fabulous artists working on Steam Driven Movie and his work is a great example of what steampunk is supposed to be. Check out his website for some more examples of his work: http://www.drgrymmlaboratories.net/
I feel like the best way to express steampunk is through film. It's the only medium that can really immerse you in the fantastical alternate realities. Both the costumes and sets need to have the right blend of technology and antiquity to really get the feel right. So here are some of my favorite examples of Steampunk movies:
The City of Lost Children, 1995:
Originally "La cité des enfants perdus," is a French film that really seems to get the genre just right. http://www.imdb.com/title/tt0112682/ "A scientist in a surrealist society kidnaps children to steal their dreams, hoping that they slow his aging process."
Wild Wild West, 1999:
"The two best hired guns in the West must save President Grant from the clutches of a 19th century inventor-villain." Just take a look at the tech in this movie to know what steampunk is.
Sexy old school garb, check.
Steam-powered wheel chair, check.
Giant robotic spider, check.
http://www.imdb.com/title/tt0472033/ "A rag doll that awakens in a post-apocalyptic future holds the key to humanity's salvation." This film really gets down into the knitty gritty details that are essential to the steampunk design:
Loved this movie BTW's.
Long post, but I have just a few more things to share with you on how main stream culture is embracing the steampunk vibe all over the place.
Hints of the "new" punk are everywhere!
So, I hope my answer was helpful :-)