A plugin is a kind of Joomla! extension. Plugins provide functions which are associated with trigger events. Joomla provides a set of core plugin events, but any extension can fire (custom) events. When a particular event occurs, all plugin functions of the type associated with the event are executed in sequence. This is a powerful way of extending the functionality of the Joomla! Framework. It also offers extension developers a way to allow other extensions to respond to their actions, making extensions extensible.
The Joomla! plugin architecture follows the Observer design pattern. The JPlugin class is derived from JObserver and provides the means to register custom plugin code with core or custom events. The JEventDispatcher class, derived from JObservable, is an event handler which calls all plugins registered for a particular event, when that event is triggered.
Prior to Joomla! 1.5 plugins were known as mambots. Mambots were able to change content only and were based on certain signal words. Since then their scope has been stretched and extended to such an extent that to enhance the meaning of the new functionality the developers decided to change the name from mambots to plugins. Joomla! 1.5 includes legacy support for Joomla! 1.0 mambots.