death metal, black metal/nordic metal, doom metal, power metal, some thrash(not counting for latter 80's thrash like metallica and iron maiden crappy thrash yet they are mentioned sometimes around extreme metal???) viking metal and sludge metal.
now all these subgenres push many a boundary in either skill or blind noise and crazyness, this is especially shown in the subgenre noisecore which is a personal favarite of mine, also grindcore, goregrind, deathgrind, shitgrind, mathcore, whirlwindgrind just to name a few that certainly push noise boundaries, and technical boundaries.
bands like blink 182 and so forth are only big in the industry because it is very easy to listen to, theres not to difficult guitar playing and very happy bubblegum eating singing, and same goes to nu-metal, a max of 4 chords are played and a load of crappy shouting goes on(yet never offensive) so this is the way for so called metal to get in the charts and shit because it doesnt get too extreme, and so idiot dumb teens can listen to that thinking there hardcore without having to make an effort to hear more inferiorly talented catagorys than pop and nu-metal. basically anything u see on totp or hear on the radio, in other words commercialized music will most probably not be extreme metal.
Yes it is a very big confusion between people wanting to know what extreme metal is and why it is not commercial, and it is very annoying having to explain over and over to people, and prob after telling u people this u will probably forget it tommorow and think slipknot is extreme metal, but anyway i hope that helped a few people out.
btw i was the one who posted that first definition(chris neaves), man that was a pretty crappy explanation long time ago aswell, so i hope this one was a little better explained
idiot: oh yeah it rocks man, i listen to korn all the time, proper black metal that is.