It's a combination western/sci-fi. The Firefly in the title is a type of space ship. This one happens to be called Serenity. The crew are basically outlaw mercenary types. They smuggle, steal and whatever else to make money, but they are the good guys and really only steal from "those that deserve it." Kind of like Robin Hood.
I've watched it from the beginning and I really like it. It's created by Joss Whedon (Buffy & Angel) and has the same kind of feel for characters and dialog. I should also point out that in my opinion, Buffy is one of the best written shows on TV right now.
KenM is correct in that it does not seem to be doing very well in the ratings. This is probably because it is a little strange. While everybody is flying around in space ships, they talk like they are in an American Western and the society seems very American Western also. So far I've really enjoyed the show and tape it every week so I don't miss it.