Prepatch

Prepatch or "RC" kernels are mainline kernel pre-releases that are

mostly aimed at other kernel developers and Linux enthusiasts. They

must be compiled from source and usually contain new features that

must be tested before they can be put into a stable release.

Prepatch kernels are maintained and released by Linus Torvalds.

Mainline

Mainline tree is maintained by Linus Torvalds. It's the tree where

all new features are introduced and where all the exciting new

development happens. New mainline kernels are released every 2-3

months.

Stable

After each mainline kernel is released, it is considered "stable."

Any bug fixes for a stable kernel are backported from the mainline

tree and applied by a designated stable kernel maintainer. There are

usually only a few bugfix kernel releases until next mainline kernel

becomes available -- unless it is designated a "longterm maintenance

kernel." Stable kernel updates are released on as-needed basis,

usually once a week.

Longterm

There are usually several "longterm maintenance" kernel releases

provided for the purposes of backporting bugfixes for older kernel

trees. Only important bugfixes are applied to such kernels and they

don't usually see very frequent releases, especially for older

trees.