From May 6 to May 8, 2011, the entire city of Waltham, MA, transformed itself for a Steampunk Festival hosted by the Charles River Museum of Industry and Innovation. There was art, dance, film, and, of course, elaborate Victorian costumes, all in the Steampunk tradition, celebrating the union of the innovation and imagination of the futurists of the late 19th century with the technological capabilities of 2011.
I first heard of Steampunk a year or so back when I was submitting manuscripts to agents and kept seeing Steampunk as a hot novel genre. And while perhaps the Steampunk term was originally coined by science fiction author K. W. Jeter to categorize writers, it has become a much larger movement.
Steampunk has captured the imaginations of artists, musicians, architects, engineers, interior designers, clothing designers... you name it. If, like me, you watch Castle, you might have seen the Steampunk episode with the antique pistols and secret society. (And, if you did see it, or want to see it, you should read this fun interview about the episode with Castle’s creator Andrew W. Marlowe.)
Defining Steampunk is slippery -- I guess I'd call it an eclectic aesthetic. According to the blurb for the book Steampunk (a compilation edited by Ann and Jeff VanderMeer), it's "Victorian elegance and modern technology: steam-driven robots, souped-up stagecoaches, and space-faring dirigibles fueled by gaslight romance, mad scientists, and oh-so-trim waistcoats."
It's any way of injecting modern-day technology into a Victorian-era, pre-Industrial-Revolution aesthetic. It's Jules Verne and Sherlock Holmes, gadgetry and beauty. It's using old typewriter keys on your computer, or fashioning a Bluetooth ear piece out of gears. It's brass, and goggles, and corsets.
Interested in learning more? This page does a nice job of collecting varied descriptions and ideas of Steampunk, and this page has some more cool pictures to look at.