Extreme Metal

[Maddolis]

Yeah it's no wonder Agalloch are one of the more popular metal bands around right now.
Atmospheric stuff is generally more slow paced and reliant on the 'atmosphere' created rather than melodic lines and such. See Drudkh as an excellent example (the album Autumn Aurora being their best, highly recommended). Technical's a little trickier to define (especially compared to progressive)- I feel that tech-death bands have tricky stuff to perform (ie fast swept arpeggios or scale passages or whatever), but their music isn't progressive or out-there in terms of the song structures or perhaps time signatures as much. Thus a band like Fleshgod Apocalypse, Hour of Penance or whoever would be tech-death but not prog-death, yet a band like Atheist could be considered either. The whole subgenre debate somewhat comes down to opinion, but 'technical' music (most commonly seen in 'technical death metal') is generally faster and more active (or more 'technically' difficult), but not necessarily progressive.

Then you have bands like Windir where the atmosphere certainly plays a part, but they're not billed as "atmospheric black/folk metal" because the band doesn't rely on the atmosphere to carry them, rather it just contributes.