It is a hard genre to describe verbally. The general essence is what Spaz said. It's a sort of science fiction version of the Victorian era where everything developed using steam power instead of transitioning to electricity and thus you get all the engines and goggles and gears. Not that those need to be prevalent in a steampunk design, but they are the most recognizable.
For some people, wearing the clothing isn't a costume, but a way of life. They wear it no matter what, not just for cons. I know a few people that do, and while they might go a little more over the top for cons and meet-ups, they still hold the aesthetic in everyday life.
Check out these forums
for more in depth stuff. The people there are very into the genre.