Extreme Metal is a catch all term alongside many other genres of Heavy Metal. The term would be used to reference bands within the Heavy Metal genres that use ‘extreme’ elements in their music, such as blast beats, very fast or very slow tempos, heavily distorted vocals, and depressing or aggressive themes within thier lyrics.
The Dictionary Definition of "Extreme" is;-
adjective, ex·trem·er, ex·trem·est.
1. - of a character or kind farthest removed from the ordinary or average:
2. - utmost or exceedingly great in degree:
3. - farthest from the center or middle; outermost; endmost:
4. - farthest, utmost, or very far in any direction:
5. - exceeding the bounds of moderation:
So, if you apply the Dictionary definition, to any and all forms of Heavy Metal, then you should be able to get an idea of what Extreme Metal should sound like.