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ExpressionEngine is the ultimate in flexibility, designed to be a platform for developers to easily extend to enable rich functionality. Like an operating system, it ships with a set of “apps” to cover the basic necessities right out of the box. We call those apps Add-Ons.
And just like an OS, Add-Ons that don’t come included with ExpressionEngine can be added to bring even more functionality to the platform. ExpressionEngine was built on a CodeIgniter foundation and includes a robust developer API to enable rapid Add-on development, and a rich ecosystem filled with high-quality Add-Ons has grown up around it.
With Add-Ons available from EllisLab for download and third-party developers, you can construct a web publishing system tailored to fit the needs of the project at hand.