There have been a number of attempts to properly define the movement, and many have fallen short. Just what is steampunk? And where did it come from? What's up with the whole "Victorian" thing? The video below is one of the best explanations to date.
So Steampunk is a movement loosely organized around retro-science fiction and fantasy expressed through the style, materials and craftsmanship of the Victorian era. Nowhere has that fantasy world been better captured than in the radical MASKS associated with the genre.
Ukrainian artist Bob Basset is the most celebrated. The author's favorite blog, Boing Boing, has done a fine job of covering Basset's work here and here.
Steampunk has definitely had an impact on fashion. The melding of fantasy Victorian with modern-day technologies - as the Victorians would have implemented them - creates an irresistible science fiction palate that is well-exploited by contemporary fashionistas.
One of the best media for Steampunk themed storytelling is animation. And the seminal animated short film in that regard (author's opinion) is "The Mysterious Explorations of Jasper Morello." A personal favorite, included below.
"A Gentleman's Duel" is another, outstanding animation - recently voted to the YouTube "recommended" list.
Finallly, Steampunk has had a profound affect on music that we're likely to see more of as this movement continues to catch on. Abney Park is the Author's personal favorite Steampunk band. They're the whole package.
When I was a boy, my father was a devoted deep-sea fisherman. He had an all-wood construction fishing boat that slept four. Among his collection of nautical instruments, he had one of those great metal and glass compasses in a wooden box with metal screws. It was mounted inside on it's own cool frame and the top of the box was lined with purple silk inscribed in gold thread. It made quiet an impression on me. He also had one of those bulky but very cool wrist compasses, with a weathered, leather strap. Steampunk makes me feel like I did then, handling his mechanical treasures. Just like I did the first time I saw Captain Nemo's fantastic ship in the early 20,000 Leagues Under the Sea film; or, watched the original Dune.