From Joomla! Documentation
JModel is an abstract class which provides the basic functionality for concrete model objects in conjunction with Joomla's MVC pattern. Models that extend the JModel class are often defined in Joomla components and define the business logic of the specific component. See MVC Development in Joomla
The Joomla MVC Design Pattern
Model-View-Controller (MVC) is a set of design patterns, that are used to separate the different layers of your application.
- The View displays the data of the model by passing it to a template. The view object can therefore access the data of the model. Joomla implements the basic functionality in the abstract JView class.
- The Model stores the data of the application, and contains the business logic. Joomla implements the basic functionality in the abstract JModel class.
- The Controller handles the user requests and evokes the model and the views to change it's state. Joomla implements the basic functionality in the JController class.