First of all all the classes are available for code study under:
\libraries\legacy\model\
It's also important to understand that these classes should be used (generally speaking) in connection with the corresponding controllers: JControllerLegacy, JControllerForm and JControllerAdmin.
JModelLegacy - is the base class for the Joomla model (model as in MVC). It will basically work as a Factory class by initializing the database driver object and the table object.
You may want to extend this class if you just want to do some basic SQL queries (any work with JDatabase) and write other business logic.