What is a Joomla Plugin?

A plugin in a Joomla website installation adds a function that triggers upon the occurrence of an event.  Like Extension of Joomla, a Plugin adds a feature/function/ability to your Joomla installation, but the Plugin is executed or runs upon a standard included event of Joomla or an event of an Extension (a custom event), depending on the Plugin's function and settings.  As stated on Joomla's documents page as of March 2014, "Plugins provide functions which are associated with trigger events".

Even though the following is for people who want to create a plugin for Joomla, and you may not be a developer or programmer, a few of the sentences at this page may still help you: Creating a Plugin for Joomla