From Joomla! Documentation
The following articles will help familiarise you with Joomla! plugins. They are a good starting point to understanding and then developing plugins.
To understand how to install and use a plugin in Joomla it is recommended to read Administration of a Plugin in Joomla!
To understand plugins better it is advised that you create a basic plugin for Joomla!.
To read on how to run a basic content plugin in a custom module or component you may have already developed it is recommended to read Triggering content plugins in your extension.
To understand the principles on which the Plugin system works Plugin Developer Overview. To then implement this in a component you have designed it is recommended to read Supporting plugins in your component.