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.
All models will extend JModelLegacy.
JModelList - is a class for handling lists of items. It provides pagination and filtering. Basically everywhere you display a list of items, you can use JModelList. All core components rely on JModelList.
JModelForm and JModelAdmin are generally associated with forms. Forms can be the the User registration form or creating and editing a record. Forms in Joomla are defined in XML files. JModelForm will load this files as JForm objects, will load form data, preprocess the form and validate it.
This is one of the most powerful classes that you can use. They will do the heavy lifting. When you don't like a specific behaviour you can override it by implementing your own code.
JModelAdmin adds some extra admin functionalities to the form:
- perform batch operations on records.
- will do ACL checks on records.
JModelAdmin will generally be used when editing a record or for batch operations, especially in backend.
JModelItem - very unlikely you will need it. It exposes just a method getStoreId() and two properties. You won't need this class, unless for as a naming thing, instead of using JModelLegacy you want to extend it when doing something basic, such as displaying a record. (my personal understanding of things).
Conclusion: Above is a general introduction to this classes, as I see them. My suggestion is to look into detail how Joomla uses them, what methods are exposed and how to use them. They are very powerful classes, and especially for CRUD operations they will help you do all the heavy lifting. After you understand what they do, you can use them and override them when needed.