What is a Joomla plug-in?
Joomla Plug-in is the smallest form of extensions used in Joomla and oriented to perform a specific function in case if a certain event in the system takes place. For example, the visual editor in Joomla is a plug-in which is activated when the system fires the call editor event . Most Joomla plug-ins are used for certain activities and manipulations of the content before it is displayed.
Joomla extensibility lies in the fact that you can install several editors and choose which of them will work on your site. The same rule applies to all types of Joomla extensions.