Although you have heard this term mentioned in this book already, you may not know what the term means. In short, Model View Controller—from now on, referred to as MVC—is a software development design pattern. MVC is an approach to separating your applications into three segments: Models, Views, and Controllers. MVC structures your application in this way in order to promote the reuse of program code.

The Model represents any type of data that your application may use. Some examples of data that your application might use would be: a database, RSS Feeds, API calls, and any other action that involves retrieving, returning, updating, and removing data.

Views are the information that is being presented on the screen to users through their web browsers. These are usually HTML files, sometimes containing PHP code that builds the template for the website. In CodeIgniter however, views can be page segments, partial templates, or any other type of page or template.

Finally, Controllers are the business logic of your application. They serve as an intermediary between Models and Views. The Controller will respond to HTTP requests and generate web pages.

However, CodeIgniter's approach to MVC is very loose. This means that Models are not required. This is for a number of reasons. Firstly, CodeIgniter's Database Library can be used in both Models and Controllers—meaning that the extra separation of Models can be bypassed. Secondly, the Model isn't tied to the database, as it is in other frameworks such as Ruby on Rails, so the Model isn't needed in this regard. Finally, if using a Model in your application will cause unnecessary complexity, then the Model can simply be ignored.

However, Models are extremely useful, even though they are optional. Models can be loaded from any Controller, so if you use a Model function in multiple controllers and you need to change the function, you only need to edit it in one place rather than in all of the controllers. Complex queries should really be put into a Model. A collection of related queries should also be put into a Model as they can be grouped together. This makes your applications simpler, and it allows you to use the functions in any Controller.