How it works

Proposed patches to go Andrew Morton
Merged into the -mm tree
Tested for a while

Eventually patches go to Linus
Merged into mainline
Released in a 2.6.x-rc patch
Tested some more

When all is working
A 2.6.x release happens

Thus
Andrew's -mm is the current development tree
Linus's 2.6.x is the (relatively) stable series