Originally Posted by RYM

Revelling in tension, despair, and dread, Doom Metal prizes atmosphere more than many of Metals other primary subgenres. Doom uses plodding tempos, repeated chords, and thick, distorted, down-tuned guitars to attempt to envelope the listener in a wash of desperate foreboding that can commonly last to the ten minute mark and beyond. Instruments generally keep to the standard guitar, bass, and drums, but it is not unusual to add keyboard passages and backdrops.

Doom vocals can be in any of the predominant Metal styles, but all share a determined attempt to invoke emotional hopelessness and mental anguish. This is further amplified by the lyrical themes of grief, depression, fear, resentment, and suffering that are hallmarks of the style.

The roots of Doom Metal are considered to have been planted in the early 70s work of Black Sabbath. In the early and mid-80s, European artists like Witchfinder General and Candlemass adopted the Doom sound as a full fledged genre unto itself, as well as American acts like Saint Vitus, Trouble and Pentagram. Their style of thick and depressive Doom remained the standard until the early 90s, when newcomers such as My Dying Bride, Cathedral, Electric Wizard, and many others began to experiment in melding Doom with several other Metal subgenres. Doom has proven to be one of the most lasting and recognizable Metal styles, maintaining a relatively small but devoted fan base in many countries.

Originally Posted by Wikipedia

Doom metal is an extreme form of heavy metal music that typically uses slower tempos, low-tuned guitars and a much "thicker" or "heavier" sound than other metal genres. Both the music and the lyrics intend to evoke a sense of despair, dread, and impending doom.[1] The genre is strongly influenced by the early work of Black Sabbath,[1] who formed a prototype for doom metal with songs such as "Black Sabbath", "Electric Funeral" and "Into the Void". During the first half of the 1980s,[1] a number of bands from England (Pagan Altar, Witchfinder General), the United States (Pentagram, Saint Vitus, Trouble) and Sweden (Candlemass, Count Raven) defined doom metal as a distinct genre.