The score (aka background music) for Fight Club was composed and produced by the Dust Brothers. (I think they're pretty famous, but the only other thing they did that I can remember at the moment is an arrangement of "mmmBop.") Back in the day, "soundtrack" and "score" were synonymous, but now score refers to the instrumental music actually used in the movie, and soundtrack is "music inspired by the film" (i.e., singers who were bribed into letting their music be used to increase profits from the film). As far as I know, there isn't a "soundtrack" to Fight Club.
That said, in my opinion the CD is pretty good as far as instrumental music goes. I can't really describe the music as any one particular style, although some of it could be called metal. But hey, you've seen the movie, you know what I mean. I don't think I'd listen to the CD over and over, but I don't regret buying it. In other words, if you know someone who has it, borrow it.
For what it's worth, my favorite tracks are "Who Is Tyler Durden?" and "Space Monkeys," which has that cool circusy music that was so funny in the movie.
Click below to buy the CD from Amazon.com.
There's also an import version that has a sixteenth track, "This Is Your Life."