(the heaviest of the heavy)
Subgenre of heavy metal, and not a very popular one at that. Doom metal, or simply doom, is characterized by simple, powerful riffs played somberly, even lethargically. Since the musicians don't bother to hurry or attempt technical feats on their downtuned guitars, the music often acquires a majestic feel and can become extraordinarily heavy, even more so than death metal, while still retaining a strong sense of melody.
The first doom band was unquestionably Black Sabbath. They wrote simple songs that a well-practiced garage band could pull off and not sound like buffoons, but Tony Iommi's genius was such that the amps still oozed power and darkness. There were a few other bands in the early 1970's with doom-ish sounds, most notably Pentagram, but Sabbath's early albums are considered the origin of the style.
Then came the late 70's, with the rise of the punk scene. Metal bands, consciously or not, emulated their musically underdeveloped contemporaries and began to speed up their sound. Doom found itself on hiatus for a while until the mid-1980's, with the development of such bands as Candlemass, Trouble, and Saint Vitus. Candlemass's 1986 debut album, Epicus Doomicus Metallicus, was the first to bring the notion of a doom "sound" to the public consciousness.
Doom finds its purest form in the early Candlemass albums. Incredibly slow, chugging bass and guitar riffs, superimposed with skillful but subdued leads and operatic vocals. This sound, aptly named melodic or "epic doom," was taken up by Solitude Aeturnus as Candlemass waned, and is still performed with great success today by the latter band. Lyrical themes are invariably depressing, but with a fantastic bent and a genuine sense for dramatic tragedy that sets the doom aesthetic apart from the merely pathetic angst found in such commercially successful genres as grunge and nu-metal.
In their departure from this archetype, other varieties of doom metal can be identified by their overlap with different musical genres. Following is a likely incomplete list of musical styles that have inspired doomy permutations:
Death metal: Known for its growling vocals and grinding riffs that aspire to brutality rather than mere heaviness. Nevertheless, some slower death metal has on occasion attained a doom-like quality: check out Amorphis's excellent The Karelian Isthmus for some epic music of a hybrid doom-death style.
Stoner rock: There's no clearly drawn line distinguishing so-called stoner music from doom metal. Basically, it's doom with a lot of groove, and is associated, justifiably or not, with those whose life pursuits revolve around the mellow green stuff. Cathedral play stoner music clearly on the "metal" side, while Kyuss, just as heavy in my opinion, tend more toward the rock-and-roll side of things.
Gothic rock: Dark imagery does not a gothic band make, nor a doom one for that manner. But the two genres already have that aspect in common, and many bands cross the line between the two during the course of their careers. Paradise Lost is probably the best example; their aptly named second album Gothic was still undeniably metal, but its lumbering riffs were interspersed with guitar hooks that wouldn't have been out of place on a Bauhaus recording. Later PL albums became increasingly subtle until any traces of a metal past had been forgotten.
Progressive rock/psychedelia: You'll often hear influences from such greats as Pink Floyd and Camel in the more musically ambitious doom bands. The shining example of prog-doom is really Anathema, which started out as a primitive doom-death outfit and matured into a highly skilled band whose mastery of atmosphere lets them create some truly beautiful music.