The MOAI Codebase consists of a number of modules, Interfaces, and an extension mechanism.
The Interfaces can have several implementations, and can be plugged into the MOAI server, by using specific configuration profiles.
Besides the code, there are also unit tests. You can run them by calling the test script in the bin directory. Most of the tests are in the form of unit tests, but there’s also a nice doctest in the README.txt file. This provides a birdseye overview of the whole MOAI system.
You can also use the MOAI Module Index for browsing the modules.