This API is split into numerous modules, each with its own collection of classes, properties and methods. Functionality to cover all aspects of app development.
The modules include Android, App, Calendar, Cloud, Contacts, Database, Facebook, Filesystem, Geolocation, Map, Media, Network, Platform, UI, XML and Miscellaneous.
Some, like the UI module, contain a huge amount of functionality, while other have just a few classes. The full Titanium API documentation is posted online, along with plenty of code examples.
In addition to this documentation there's loads of tutorial material. This includes an impressive list of videos suitable for all levels, from beginner to expert.