In the past 30 years the music known generically as 'Metal' has split into a wide variety of forms, including death, black, thrash, speed, progressive and melodic death metal. These six genres are considered to stand out from the rest, and require further explanation. Generally, bands within these genres have remained under the confines of the term 'underground'. Underground bands are rarely, if ever, heard on radio or seen on television. Album sales are considerably lower than mainstream acts. It should also be known that many of these bands will never break into the mainstream due to the very nature of the music itself: loud, fast, and hate-filled.
Death metal can be summarised as heavy and thick guitar sounds with low, guttural vocals mixed in with blasting drums1. The vocals range from low-end growls to 'demonic screaming'. The lyrics tend to be hate-filled, and some are fairly grotesque. Many death metal bands seem to be competing for the most repulsive lyrics and album covers. One of the most important bands, and attributed originators of this genre, is Death, whose front man, Chuck Schuldiner, tragically passed away a few years ago.
Black metal tends to be dominated by high, screaming, ear-piercing vocals. A good example of this can be seen in Cradle of Filth. The lyrics are often, though not always, anti-Christian. Blastbeating drums and synthesisers are commonly used, although the latter certainly isn't exclusive to all bands in the genre. There is a lot less string-muting2 in black metal, compared to death metal, as well as a lot more raw production3. This is not always the case; bands such as Dimmu Borgir are important examples of black metal with good production. Alongside Black metal there is a sub-genre called blackened-thrash metal. It is, as the name says, a combination of the black- and thrash-metal styles. The use of keyboards is virtually removed, and the drums are generally slightly slower, though they still retain speed. Examples of exponents of this style are Immortal and Susperia.
Heavy distortion with insanely-fast solos, many of which are very intricate, make up the basis of thrash metal. The guitars tend to be standard tuned4. Riff styles combine a variety of styles; melodic at times, yet brutal and fast as well, and even blues-based solos are thrown in from time to time. Thrash music tends to revolve around the darker side of life, but there are fewer anti-Christian messages than in black metal. Old Metallica5, and Slayer are prime examples.
Speed metal is simply just that: fast. The guitars are fast, the bass is fast, the drums are fast, and the vocals are, well, fast. The song writing tends to be well-structured and technical, generally requiring supreme musicianship. Speed metal has been around for quite a while, certainly since the early 1980s - a relatively long time for a metal genre. Testament, who have been on the scene for most of that time, are possibly the best-known example of speed metal in action.
'Prog' metal takes influence from several different genres of music. Classical, death metal, thrash, blues, and folk are regularly intertwined to form a technical and challenging musical form. In fact, it might be called the thinking man's metal as there is often an abundance of strange timings and much experimentation with sounds takes place. A prime example of progressive metal is Opeth.
Melodic Death Metal
MDM utilises many styles, much like that of prog metal. Guitar harmonies and melodic interludes are a staple of the genre, which has drawn comparisons with classical music. To further this, many bands have used parts of classical works in their songs, including bands such as Children of Bodom, who used parts from pieces by Mozart in their songs. Vocals can be like that of black metal, but they can also be clean, and even whispers at times; many times interchanging between the two. Another name for this genre is Gothenburg Metal, named after the city in which it allegedly spawned through bands such as In Flames and Dark Tranquility.
With most bands ever keen to expand their style and repertoire, it is likely that the boundaries between these genres will become increasingly blurred. The styles listed above are reference points only, and by no means tie a band down to a 'label' - that would be against the spirit of metal!