Plugins are small run-time enhancements. They are simple to create and free to be distributed. The plugin architecture is designed to enable you to affect the output of ExpressionEngine in a wide variety of ways.

Modules are larger systems that can be integrated into the ExpressionEngine control panel. Almost anything that can be achieved as a stand-alone application can be designed as an ExpressionEngine module.

Extensions are “hooks” in the system that allow you to pass information and add functionality to existing interfaces (like the Publish page for example).

Please see the Developers area for more information.