Since Agent didn't cover doom metal, I'll field that one
Doom metal takes most of its influence from the grandpa of all metal bands, Black Sabbath. Music style involves slow plodding drums and riffs, with vocals being either growly-death or clean baritone/bass male vocals along with the occasional mixed-in female vocals (growly male vocals and clean female vocals combined is referred to as "Beauty and the Beast" style). Lyrical style is gloomy and usually filled with dark imagery and despair.
Notable bands include Candlemass, Paradise Lost, My Dying Bride and older Anathema. Of all the big metal subgenres, I would say doom comprises the smallest portion of the overall metal family (probably because metal is most associated with blazing fast guitars and shrill vocals, which is the complete opposite of this)