Your templates automatically get the tg.tg_js variable, and MochiKit is automatically made available as a static file from the server. When you load MochiKit this way, you’re getting everything from the MochiKit modules. MochiKit includes:
MochiKit includes so much functionality, that this module breakdown makes it much easier to follow (and work with, if you’re making improvements to it).
MochiKit’s documentation is very good, so there’s no reason to duplicate it here. To get an idea what each module has to offer, take a look at the introductory material at the top of each module’s documentation.
It is worth skimming through the function reference as well, because each module has features that will make your life easier. For example, you know from the synopsis that MochiKit.Base offers features like map and repr... but did you know that it also has functions for putting together and parsing HTTP query strings?
On the last page of the 20min wiki tutorial you find an example how MochiKit can be used to pep up a TurboGears application.