Services_Yahoo is split up into chunks that group similar functionality. Currently there are chunks for the interfaces to Yahoo Search and to the Content Analysis functions.
Each chunk basically contains two top-level element in the source tree: A .php file and a directory that holds the implementation of the chunk. The file is a class that contains only one method which implements that factory pattern. This method dynamically loads the right file from the related directory and returns an instance of the class that implements the desired function.
Due to this modular layout Services_Yahoo can easily be extended when Yahoo adds new functions to their API.
Also the automatically generated API documentation should help you.