Filk is a type of music, loosly related to Science Fiction. Nick Smith wrote a description of filk, Tom Smith wrote a a different description, and Michael Liebmann wrote another description, but there is no simple agreed-upon definition. ("Folk music of the Science Fiction/Fantasy fan community" probably comes closest.) (The jargon one is wrong, filk can be to new music and much of it is not humorous.) Lee Gold's An Egocentric and Convoluted History of Early "Filk" and Filking is also available.
Many SF cons have some kind of filk, ranging from entire filk conventions to giving the filkers a room so they will shut up. (At least one con learned the hard way not to deny the filkers a room they had been promised -- it was hard to do much with a bunch of filkers singing in the ops room.)
LAFA (Los Angeles Filkers Anonymous) gets together once a month in the Los Angeles area to filk.
The newsgroup rec.music.filk is bidirectionally gatewayed to the fidonet filk echo. The fido "Topic Cop" Kay Shapero kay.shapero@salata.com tries to keep discussions on topic (puns are allowed, as is almost anything in song) and posts several informational postings occasionally, which are now also available on the web at: http://www.hooked.net/~kirib/filkfaq.html. The previous newsgroup alt.music.filk has mostly been abandoned.