The Model classes are PHP classes responsible only for representing database entities in memory. For example, the articles table stores article information in the database; there is a corresponding Model class called Article (see classes/article/Article.inc.php) and DAO class called ArticleDAO (see the section called Data Access Objects [DAOs]).
Methods provided by Model classes are almost exclusively get/set methods to retrieve and store information, such as the getTitle() and setTitle($title) methods of the Article class. Model classes are not responsible for database storage or updates; this is accomplished by the associated DAO class.
All Model classes extend the DataObject class.