Doom Metal (allmusic, wikipedia) is stylistically a bit more disparate than some of the other subgenres, but Doom is characterized by its goal of setting a mood (any guesses as to what mood?).  It isn't necessarily as dirge-like as I expected based on the name of the subgenre (at least, not the samples I listened to), but it is dark and foreboding.  I think it's also a bit more musically accesible than some other genres (e.g., Death Metal); rather than focusing on musical intensity or vocal aggression, for example, Doom Metal is really all about the enveloping tone of dread the music is constructed to create.