The following articles will familiarise you with Joomla! plugins and are a good starting point to developing your own 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!.
It is also advised 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.