I know, I know, that's not a very helpful description. Unfortunately it's the only definition that I know of that is even remotely accurate. For the benefit of those who haven't a clue what I'm talking about at all, let's try again. Filk is the folk music of science fiction fandom. Except that that ignores all the SF-related music produced by people who aren't fans, and the non-SF-related parodies produced by nearly everyone, and...
You begin to see the problem?
"Filk" was in fact a misprint for "folk" originally. Legend has it that the world first appeared as a typo in a convention programme listing, which intended to promise that "folk singing will occur as usual in the lobby." The earliest recorded usage of the term (unless someone can produce a copy of that programme book) is an article written by Lee Jacobs in the 1950s, noted in Harry Warner's fanhistory "A Wealth of Fable."
But what does it mean? Unfortunately, filk means different things to depending on who you talk to and when you talk to them. Sometimes it refers to a parody of a known song, usually but not always with an SF or fantastic leaning. Sometimes it refers to an original song of SF or fantastical nature. Sometimes it refers to a song written or performed by someone who may never have heard of the term before.
For me, as I said, it's what I point to when I say "This is filk." As far as I can tell, any other definition is hopelessly inadequate, because it misses out too much. Even calling it "SF folk music" gives the wrong impression: it misses out people like Les Barker, whose folk-poem parodies often contain SFnal or fantastic imagery, and it implies that the folk music style is the only one used, despite the proliferation of hard and soft rock, hymns, classics and partsong styles that blossom in the filk communities. OK, granted that the last one is my fault, and the vast majority of performed filk consists of one singer and his or her guitar, but it's still incomplete.
To be honest, the only way to really understand what filk is about is to experience it. Preferably late at night at an SF convention, as filk circles tend to be, when you are about as tired out as you can get and some kindly soul is passing bottled invigoration around the room. Failing that, the page above has links to some of my own filk songs (hopefully with attached MIDI files, depending on how much time I have had recently and whether or not everything works properly), as well as links to other filk sites around the web. Do go and have a look, because I can pretty much guarantee that something will catch your fancy, somewhere within filkdom!