Death Metal is one of the most controversial and misunderstood genres of music today. I say misunderstood meaning not only that people unjustly scrutinize it, but also mislabel it as something else. To start off I'll outline some of the different sub-genres of Death Metal.
NOTE: I will not cover every band. Don't get upset when your favorite band doesn't show up on the list.
OLD SCHOOL DEATH METAL:
Some may call it Classic Death Metal, others may call it Old School Death Metal, but it amounts to the same thing. It started with bands like Venom and Possessed who played fast, angry, and dark music, Possessed, going further then Venom. An off-shot of Thrash, Death Metal is more aggressive and in some cases faster. The lyrics are usually (but not limited to) violent, covering topics like war, death, and other gory subjects. Old School Death Metal vocals were often high pitched screams or groans, while the guitar work emphasized fast tremolo picking. Although some old school death metal bands utilized the death growl that made death metal so distinguishable, it wasn't used as much among the first death metal bands. It was made more popular in...
BRUTAL DEATH METAL:
Brutal Death Metal essentially took the aspects of Death Metal that gave it its name and expanded on it. Heavier riffs, more violent lyrics, and deeper, more guttural vocals. The drumming incorporated more blast beats as well and emphasized a more complex style of playing. The lyrics are usually more graphic then classic death metal.
MELODIC DEATH METAL:
-At The Gates
Melodic Death Metal is pretty self-explanatory; Death Metal that has melodic elements, often combining elements of NWOBHM with death metal. Originating in Gothenburg, Sweden, this sub-genre is sometimes called Gothenburg metal. Bands like At The Gates, Dark Tranquility, and In Flames are credited with starting the genre. Many bands incorporate the use of keyboards and synthesizers as well.
BLACKENED DEATH METAL:
-Keep of Kalessin
Once again, Blackened Death Metal is pretty self-explanatory. Death metal with black metal elements. Most of these bands focus on Satanism and occultism like its black metal roots. I won't pretend to be an expert on this genre because I don't listen to a lot of it, however, it is usually easy to distinguish from other death metal bands. The vocals usually inherit a style like its black metal influences while maintaining the instrumental styles of death metal.
Beginning to see a pattern yet? Goregrind/Deathgrind takes grindcore and death metal and puts them together. It takes the intense aggression and speed from grindcore and combines it with the complexity of death metal. Fewer guitar solos, often shrieking vocals mixed with death growls, and shorter songs. Some consider Goregrind and Deathgrind to be separate genres, however I once again am not too familiar with the genre.
-My Dying Bride
Don't worry, this is the last "take-two-and-put-them-together" genres I'm covering in this article. The slow tempo and depressing atmosphere that doom metal brings is combined with the deep guttural, growls and the double bass of death metal. Once again, I won't pretend to know more then I do about this genre so I'll stop here.
TECHNICAL DEATH METAL:
Sometimes called Progressive Death Metal or Tech Death, Technical Death Metal often use complex song structures and time signatures. They take death metal and draw upon further influence from jazz, classical, and progressive music. Most tech death draws heavy influence from later Death albums.
-Born of Osiris
-Job for a Cowboy
Deathcore originated with metalcore being fused with death metal. The main staple for Deathcore are it's (often simplistic) breakdowns which utilize an often simple palm muted rhythm that follows the double bass of the drums. Vocals range from high pitched shrieks to low growls and often use the inward "Pig Squeals." Most Deathcore bands play in low drop tuning. Many Deathcore bands utilize technical playing styles and odd time signatures, which cause many Technical Death Metal bands to be confused for Deathcore, and vice versa.
Many don't classify Djent as a distinct genre, often lumping Djent bands into Technical Death Metal or Metalcore genre, however it has a very distinct sound. The name comes from the description of the guitarists' tone. "The onomatopoeia of a heavily palm muted distorted guitar chord which is usually played as but not limited to a 4 string double octave powerchord, and as a result sounds much more metallic and sonically present than a 'chug' 'chugga' or 'djun' per se..." - Bulb (a.k.a. Misha Mansoor or Bulb and Periphery). Djent relies heavily on breakdowns, however they apply them with the use of polyrhythms and unusual time signatures. Also, it is usually easy to hear the "djent" sound in the music itself.
I hope this helps bring a deeper understanding of the sub-genres of death metal.