here i want to write a bit about doom music, as a huge part of the music i listen to can be counted to this category. in general, it can be characterized as slow, heavy music with dark lyrics, dealing with depression, death, despair, gloom, dying, doomed/unhappy love, and despair. it appears in various shapes, blended with many different genres, including death metal, ambient, noise, gothic music and jazz. even though non-metal doom music does exists, most of doom music is probably metal.
(note that i like all the bands i mention here quite alot, except probably the older stuff of celtic frost which, i have to admit, is mostly unknown to me.)
one of the more traditional doom metal bands is candlemass, with their 1986 album epicus doomicus metallicus being a major landmark in the doom metal genre. the album is opened by the song solitude (“please let me die in solitude”) which is, in my opinion, one of the best doom songs.
an important crossover doom genre is death/doom which is, surprise!, a crossover between death metal and doom music. usually a bit faster than other doom styles but still being way slower than usual death metal, it features death growls, sometimes mixed with clean vocals, and certain playing styles from death metal such as fast double bass kicking. two bands in this genre i like are swallow the sun—one of my most favorite bands; see here— and novembers doom. notably are also the older works of bands as anathema (see here), now playing atmospheric metal, the gathering (see here), now playing progressive/alternative/trip rock, and sentenced—another one of my most favorite bands; see here—, who later turned to more gothic doom or plain heavy metal, before disbanding 2005.
out of death/doom, another doom crossover called funeral doom developed, by slowing down the music, resulting in something which is as slow as a funeral march, hence hinting where its name comes from. besides the lack of speed, the music is even heavier, but also features more ambient like parts. two bands in this category are tyranny and unholy (see here).
related to funeral doom is drone doom, abstracting even more by reducing to a kind of minimalistic music, mainly consisting of a stream of drones, i.e. very long lasting notes, while often lacking traditional music structures like rythm or melody, with vocals often so distorted that they are barely understandable. the songs usually last very long, from something between ten and twenty minutes to sometimes filling a whole cd with one song. two important bands in this genre are sunn o))) and boris (see here).
another doom genre which is pretty detached from metal is doom jazz or noir jazz, a blend of doom music and jazz. the only example i know is bohren & der club of gore, a german band who started mixing jazz with hardcore and ambient, resulting in some kind of doom music. later, when their guitar player left, they replaced him by a saxophonist, turning the band completely into a jazz ensemble. their current style is a very dark kind of jazz, without vocals, somehow almost more ambient, somehow similar to funeral doom.
a lot of other more experimental acts exist, like early the 3rd and the mortal (see also here) stuff, which can be described as experimental/atmospheric doom, or the zürich based metal legend celtic frost, whose last album monotheist could be in parts counted as a certain kind of avantgarde/experimental doom.
another, more “standard” doom metal approach is gothic doom, a mixture of doom with gothic metal. my favorite in this area is type o negative, and in particular their most depressing album world coming down. some bands which i would also count into this category are tiamat, lake of tears and later sentenced. tiamat evolved more to gothic rock and had included also different influences besides gothic doom. sentenced began with death/doom, switching to more gothic doom and finally more to a kind of heavy/gothic metal. lake of tears, whose origin lay more in the death/doom origin, went through a change between several different genres; the albums which fit best as gothic doom are headstones, a crimson cosmos and forever autumn, all being somehow different to each other.