Filk is probably best described as the folk music of the science fiction fandom community. The word itself was coined as a typo in a fan publication. Originally, most filk music consisted of parodies, or new lyrics (on science-fictional or fantastic subject matter, with the subjects of the computer industry, science in general, and the space program being included under the science fiction banner) written to existing tunes. Nowadays, there are plenty of professional musicians in the filk community, often writing their own tunes (which then generally become grist for some parodist's mill), and selling tapes of their performances. The subject matter for parodies now includes politics, popular culture (you don't want to know how many filks I've heard about John Wayne Bobbitt, or the bondage subculture), and just about anything that a filker somewhere in the world is interested in. (Did I really just bundle bondage in with "popular culture"?)
Over the past two or three decades, SF fandom has been growing to such an extent that splinter fandoms are forming, and specialty convetions devoted to filk, costuming, and even specifically to written SF, can be found. I handled publications (program book, signage, badges, etc.) for Contata, the 1994 instance of the Floating East Coast Filk Convention.
This is not an exhaustive list of my filk work. There's stuff I haven't finished yet, and stuff I have finished, but would rather not see the light of day, and stuff that has seen the light of day, which I wish hadn't ("Banned From Egypt" is one of these, but enough people have asked me for the lyrics that I figure I might as well include it here), and stuff I just haven't gotten around to HTMLifying yet.