The implementation of a "Models Tier" is a huge part of any MVC framework. It represents the data of your application, and most applications are useless without data. Magento Models play an even bigger role, as they typically contain the "Business Logic" that's often (right or wrong) relegated to the Controller or Helper methods in other PHP MVC frameworks.
The following is part of a longer series about Magento aimed at developers familiar with PHP MVC development. While each article can be read stand alone, each article does build on concepts covered in previous articles. If you're confused, be sure to catch up on the older stuff first.
Traditional PHP MVC Models
If the definition of MVC is somewhat fuzzy, the definition of a Model is even fuzzier. Prior to the wide adoption of the MVC pattern by PHP developers, data access was usually raw SQL statements and/or a SQL abstraction layer (such as AdoDB). Developers would write queries and not think too much about that objects they were modeling.
In 2009 raw SQL is mostly frowned upon, but many PHP frameworks are still SQL centric. Models will be objects that provide some layer of abstraction, but behind the scenes developers are still writing SQL and/or calling SQL like abstraction methods to read and write-down their data.
Other frameworks eschew SQL and take the Object Relational Mapping (ORM) approach. Here, a developer is dealing strictly with Objects. Properties are set, and when a save method is called on the Object, the data is automatically written to the database. Some ORMs will attempt to divine object properties from the database, others require the user to specify them in some way, (usually in an abstract data language such as YAML). One of the most famous and popular implementations of this approach is ActiveRecord.
This definition of ORM should suffice for now, but like everything Computer Science these days, the strict definition of ORM has blurred over the years. It's beyond the scope of this article to settle that dispute, but suffice it say I'm generalizing a bit and probably asking to be jumped by a couple of Java developers when I leave the house.
