What are the differences between the -next tree and the -mm tree?

Hi, I'm sorry to bother you to ask some questions about -next tree and -mm tree. I have read some documents about kernel trees, but I don't know the difference between the -next tree and -mm tree. In this post: http://lists.kernelnewbies.org/pipermail/kernelnewbies/2013-October/009145.h... It seems that the -mm tree has been substituted by the -next tree. Besides, I can't find -mm tree in anywhere. So, could you explain the difference between the two trees. Regards, HaoLee