These plugins are behind the buttons found beneath your editor. They only run when an editor plugin runs.
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The search component uses plugins to control which parts of your Joomla! site are searched. You may choose to turn off some areas to improve performance or for other reasons. Many third party Joomla! extensions have search plugins that extend where search takes place.
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Two new plugins are available in 1.6 but are disabled by default.
System plugins operate every time a page on your site loads. They control such things as your URLS, whether users can check a "remember me" box on the login module, and whether caching is enabled. New in 1.6 is the redirect plugin that with the redirect component assist you in managing changes in URLs.
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Content plugins run when specific kinds of pages are loaded. They do things ranging from protecting email addresses from harvesters to creating page breaks.
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Components are larger extensions that produce the major content for your site. Each component has one or more "views" that control how content is displayed.In the Joomla! administrator there are additional extensions suce as Menus, Redirection, and the extension managers.
Modules are small blocks of content that can be displayed in positions on a web page. The menus on this site are displayed in modules. The core of Joomla! includes 17 separate modules ranging from login to search to random images. Each module has a name that starts mod_ but when it displays it has a title. In the descriptions in this section, the titles are the same as the names.
Templates give your site its look and feel. They determine layout, colors, type faces, graphics and other aspects of design that make your site unique. Your installation of Joomla comes prepackaged with four templates. 
Plugins are small task oriented extensions that enhance the Joomla! framework. Some are associated with particular extensions and others, such as editors, are used across all of Joomla!. Most beginning users do not need to change any of the plugins that install with Joomla!.