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Model-View-Controller

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CodeIgniter is based on the Model-View-Controller development pattern. - -MVC is a software approach that separates application logic from presentation. In practice, it permits your web pages to contain minimal scripting since the presentation is separate from the PHP scripting.

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The Model represents your data structures. Typically your model classes will contain functions that help you -retrieve, insert, and update information in your database.
The View is the information that is being presented to a user. A View will normally be a web page, but -in CodeIgniter, a view can also be a page fragment like a header or footer. It can also be an RSS page, or any other type of "page".
The Controller serves as an intermediary between the Model, the View, -and any other resources needed to process the HTTP request and generate a web page.

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CodeIgniter has a fairly loose approach to MVC since Models are not required. -If you don't need the added separation, or find that maintaining models requires more complexity than you -want, you can ignore them and build your application minimally using Controllers and Views. CodeIgniter also -enables you to incorporate your own existing scripts, or even develop core libraries for the system, - enabling you to work in a way that makes the most sense to you.

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CodeIgniter User Guide Version 2.0.3

Model-View-Controller