The only familiarity I have with the term is from a comic by Chris Bachalo with the term as it's title. What's this steampunk fashion all about?
Steampunk is more than a fashion; it actually started (self-consciously, at least) as a subgenre of science fiction. The gist of it is interpreting a world with advanced technology through the lens of Victorian Era culture and imagination. The time period itself is played with pretty loosely; there's a faction of steampunk that focuses on the American West as inspiration known as Weird West. It's also splintered to include other eras (e.g. dieselpunk is the same thing, but using the 1930s and 40s as the stylistic inspiration).
On a less abstract level, it's a beautiful marriage of goth and nerd.

Some bigger cities have steampunk meetups and excursions; one of the more active people in the scene local to me organizes a time traveler's excursion to the Renn fair every summer (so people dressed like sci-fi Victorians "time travelling" to explore the Middle Ages). It's become more visible at sci-fi conventions over the last few years too. People tend to express it based on their own talents: crafting props such as weapons or machina, creating clothing, writing stories, drawing, roleplaying, etc etc. It's not uncommon to have a persona, and base one's costumes on that concept.