Components
A joomla custom coding component is the most extensive add-on. This typically adds completely new, or different, functionality to your site and extends the overall site possibilities. A component handles data manipulation as well as input and storage into the database. A joomla coding component on most sites occupies the main area of the website layout and is the primary focus of the page.
Modules
A joomla custom module is typically considered to be an add-on to the site that extends the functionality of another part of the system. It usually has fewer options that are configurable by the end user and most of the time does not handle any storing of information into the database. A module usually occupies a secondary portion of the web page and is not considered the primary focus of a page.
Plugins
A joomla custom plugin Plugin will manipulate output already generated by the system. It typically does not run as a stand-alone piece but takes data from other sources (i.e. the content) and manipulates it before outputting to the user window. A plugin typically does not display directly to a user but does it's work behind the scenes.
