Back in the ancient times of Mac OS X 10.3 Panther, Apple began introducing us to a new system for identifying file types: Uniform Type Identifiers. Ever since then, the concept has become slowly more pervasive, with most of the file type APIs in Leopard accepting UTI strings.
It's not worth me going into detail on how it all works because Apple's documentation covers pretty much all you could want to know. But what can be overwhelming is the sheer size of the system. OS X itself declares a pretty vast range of types and wrapping your head around how they all fit together is hard. So as a means to simplification, I proudly present a nice diagram of all the important types.
Note that this diagram is so far constructed only from what Apple has declared in their documentation. I'm sure there's plenty more important ones that are commonly available on OS X, so please give me a shout as to what they are!