Doom is the genre that worships Black Sabbath; all things doom trace a direct line back to them.
Subject matter is typically dark and gloomy stuff: death, demons, bad places, bad people, hopelessness, etc.
The music is, like Black Sabbath, blues based but crucially includes the tritone. Doom is usually medium or slow paced, but not always. Low end via drums and bass are essential components, where in lots of other metal the bass is effectively nonexistent or creatively secondary. Guitars are typically down-tuned one or more whole steps. Doom is meant to be listened to LOUD, so that the music moves so much air that it feels suffocating, pressing all around you with tangible force. Or at least, as close as you can get before you actually start losing your hearing or breaking windows.
First came Sabbath just being Sabbath. Then we had some bands that kind of were Sabbath-like, but were also kinda arena rock/power metal-y; bands like Witchfinder General, Trouble and Pentagram. Then Scott “Wino” Heinrich eventually came along, realized he was born like 15 years too late and started trying to rectify that with a series of bands that continues to this day, including his first band, The Obsessed and his second band, Saint Vitus (the title of this album is Born Too Late, ffs). Wino is a key figure in doom metal, a true Defender of the Faith; any metalhead who sees him in public should thank him for his life’s efforts and offer to buy him a beer or fire up a joint, IMO (I would).
You can hear the ultra-slow drums there. The long, drawn out guitar riff is still only a few notes/chords, but it takes like ten seconds to happen instead of one-and-a-half. The melancholy, Ozzy-esque vocals. This is proto-doom, IMO, not quite fully formed yet.
Then the whole desert/stoner thing started kicking up dust (think Kyuss) and before we could cough, Sleep happened. Holy Mountain, did Sleep happen!
So now with Doom as a full-fledged genre, we started to see some ideas coalesce around the term: Sabbath influence, reverence for analog recordings, old amps, old guitars, old effect, etc. A love of marijuana. Near universal attempts to make music that was “heavy” sounding musically.
Doom bands often seem to need at least one album to get warmed up, get their feet under them, etc. The UK’s Electric Wizard fit into that category, delivering two landmark albums after a decent but not-very-exciting debut: 1997’s …come my fanatics (my favorite of theirs) and 2000’s Dopethrone. These two albums set the bar impossibly high for just how low music could be and remain seminal 2 decades on; Funeralopolis is pretty much Doom’s perfect song just like Master Of Puppets is Thrash’s perfect song.
From there, things just kind of took off. Take the basic blueprint of Black Sabbath’s music, twist it up and smoke it and call it Doom.
Another notable band/album from the early days, one that took me a few years to finally acquire back in the mid-2000, is Acrimony - Tumuli Shroomaroom. Awesome stoner/doom, but since they only released this one full length many overlook them.
Note that desert and stoner often walk the line between “rock” and “metal”; for the most part nobody gives a shit except people who want to argue like it means something.
Anyway, after that thing shave kept evolving. There’s actual stoner doom, (bands like Bongzilla and Bongripper and Seedeater, where everything is related to pot), there’s classical doom (like Candlemass), there’s Satanic doom, black doom, death doom, drone metal, etc. Even sludge metal traces it’s roots directly back to Black Sabbath (and I love me some sludge!). Oh, and certain locales also have their own special flavor of doom: southern US doom, Pac NW doom, UK doom, etc.
Doom is one of my favorite genres. I can offer plenty of recs including new bands like Whores Of Tijuana and bands that are still active after long careers, like Sleep - Marijuanaut’s Theme.
Oh, and then there’s new bands that sound like they’re old bands, like Orchid.
That’s a pretty good start, I reckon. Let me know if you wanna talk more about Doom, eh.