Basically stoner metal is ripped off Black Sabbath riffs tuned down to C or lower with a heap of dirt on bass and guitars and lyrics about either weed, demons or witches/wizards. Songs usually turn in to a jam towards the end. Check out Kyuss for an example.
For doom, tune lower and add more dirt, maybe slow down to 120bpm and add the word bong/wizard/funeral to the band name. Beards are almost essential as is black clothing. Everywhere you go there should be a fog of cannabis smoke. Check out Electric Wizard for an example.
For sludge metal tunings can be higher, up to drop D. Basically just play early thrash slowed right down with vocals kind of slurred/growled. Check out Eyehategod for an example.
Then you've got yer sub genres, funeral doom, blackened doom, drone doom, death doom etc, etc