Black Metal: Black metal finds its roots in bands such as Venom and Bathory, and is often characterized by an inherently evil tone and a raspy vocal style (Quorthon of Bathory may or may not be the originator of this vocal style, but once you've heard him, many black metal vocalists will sound very similar to him, for what that's worth). The earlier bands focused on minimal instrumentation, as represented by Darkthrone, early Mayhem, and older Immortal, while another group of bands explored a more symphonic, keyboard-driven style (Emperor, Cradle Of Filth, Dimmu Borgir) and still others have migrated towards an eclectic, avant-garde direction (Arcturus, new Mayhem). Norway continues to be the primary exporter of black metal bands, though other countries have contributed some quality bands as well.
Death Metal: One of the more extreme forms of metal, death metal is basically an offshoot of thrash, with less melodic riffs and a low, growly, often almost unintelligible vocal style that at its best has been described as "cookie monster vocals". The early nineties saw the initial rise of death metal in places such as Florida ( Death, Morbid Angel, Deicide, Obituary, and others) and Sweden (At The Gates, Entombed, and others), and is still quite popular today. A second form of death metal is the so-called Gothenburg style, named for the Swedish city where innovators such as Dark Tranquillity, In Flames and Night In Gales reside. Here, the vocal style is similar but the musical style is much more melodic, occasionally bringing to mind an Iron Maiden.
Doom Metal: Doom metal can be described with a single word -- slow. Ponderous, ultra-heavy riffing and (usually) melodic vocals dominate this form of metal. Black Sabbath are, of course, the fathers of metal in general, but their early work concentrated on slow riffing and thus is the primary influence on doom metal bands. Candlemass deserves mention for rejuvenating the genre in the eighties, and their first four albums are regarded as classics in the field. A subgenre of doom metal is doomdeath, which combines the slow pace of doom with the low, growly vocal style of death metal. Three British bands, Paradise Lost, My Dying Bride, and Anathema, are generally regarded as innovators here.
Grindcore: Made from the remnants of thrash and other crossover attempts, Grindcore fused the death metal vocal style with high energy hardcore riffing using chromatic and counterpoint compositional techniques to create streams of tonal motion, or divisions of sound into abrupt striking strum, which "grind" against one another with a primal direction in phrasing based on the rhythm of a central pair of themes. The most famous bands of this genres are Carcass, Napalm Death, Brutal Truth, etc.
Heavy Metal: The beast that started it all. Heavy metal went through several stages including excesses of commercial stadium rock before returning to its roots in alienated and rough but majestic music. The term was a quote referring to Led Zeppelin's music as "the music sounds like heavy metal falling from the sky". Heavy Metal today has become a standard rather than a genre itself. Bands like Iron Maiden belong to the sub-genre of Heavy Metal called NWOBHM (New Wave Of British Heavy Metal), a metal movement started during the 70's.
Industrial Metal: Industrial music, in its earlier form, comprised of making music with non-musical instruments. Industrial metal takes this idea and furthers it, usually by complimenting metal guitars with samples, external sound effects, and (often) processed vocals. Ministry is an acknowledged pioneer in the field, and one of their disciples, Skrew, also deserves mention as a prime influence and sound-alike for many bands in this genre. On the somewhat more mainstream side of this genre are bands such as Marilyn Manson, Nine Inch Nails and Rammstein.
Power Metal: Power Metal is the new name to the genre often called Heavy Metal. Here the music is melodic and the lyrics have some meaning. Bands like Nevermore, Iced Earth, Stratovarius, Helloween, Demons & Wizards, etc., fall into this category.
Progressive Metal: Initially, progressive metal bands fell into two camps, the first being bands who incorporated quirky time signatures and atypical riffing into metal, and the (related) second being metal bands strongly influenced by 70's progressive rock bands such as Yes, Genesis, etc. Watchtower and Thought Industry are two excellent examples of the first group, while Dream Theater remains the most well-known of the latter group, as well as the single band most fans point to first when referring to progressive metal. In later years, melodic progressive metal bands have blossomed, many of them not quite as technically oriented as the genre originally defined. Indeed, a lot of these bands can be best described as some mixture of Dream Theater, Queensryche, and Fates Warning, which indeed places these three bands as major innovators in the field. On the extreme end of progressive metal is technical metal, where the musicianship and songwriting variance is placed at an even higher premium. This is demonstrated by bands such as Cynic, and Spiral Architect.
Speed Metal: Speed Metal is a fast type of metal which uses palm muting as a strumming technique to produce bursts of alternating rhythmic emphasis. Topics like war, pollution, nuclear weapons and corporate domination are sung of in either a male bass vocal or shouted in a riot style chorusing. While this music is highly complex and often inventive in structure, it remains roughly within the confines of rock-based mainstream music and passes its technique on to the underground death metal, thrash and grindcore to follow. Slayer are the pioneers in this field.
Thrash Metal: Thrash metal is generally characterized by a fast pace, a staccato, chunky guitar riffing style, and aggressive vocals. Metallica's Kill 'Em All, released in 1983, is arguably the first true thrash album, with healthy thrash scenes sprouting in the USA (particularly the San Francisco area). By the early nineties the genre was a bit oversaturated, and in later years fewer bands played the style, but it's still a viable style today, with veteran bands such as Megadeth, Testament, and Pantera, among others, still producing quality thrash albums.
NOTE: Most of the information on this page was taken from ANUS and BNR Metal Pages.