Add-On is the general term for any optional script that extends Blender's functionality. They are found in the Add-Ons tab of the User Preferences window. This tab allows to install, enable and disable Add-Ons. Blender comes with some useful Add-Ons already, but you can also add your own, or any interesting ones you find on the web. The Scripts Catalog provides an index of Add-Ons that are included with Blender as well as listing a number of external Add-Ons.
