This is primarily an update release. This is a build for Minecraft 1.4.4 (which is currently in pre-release).
Note that this does NOT contain the celestial mechanics or other thing which were streamed earlier. Check out the change log for what it DOES contain. 😛
To recap: there is only one download (found at the bottom of this post); This file is the universal install for client and server; it no longer goes in the jar.
Mystcraft now goes in the mods folder.