Joomla! plugins serve a variety of purposes.They are like hook of Joomla!. As modules enhance the presentation of the final output of the Web site, plugins enhance the data and can also provide additional, installable functionality
The eight different types of plug-ins include the following:
❑ Authentication — Provides user authentication functionality so that additional methods may be
added. Some of the authentication plug-ins included with Joomla are LDAP, OpenID, GMail,
❑ Content — Allows content to be modified after it is retrieved from the database and before it
is sent to the user. This allows content to be modified (for applications such as email address
cloaking to prevent spam) or supplemented with additional information (such as item rating,
user comments, and page navigation features).
❑ Editor — For content editors such as TinyMCE and XStandard Lite.
❑ Editor button — These plug-ins extend the capabilities of the content editor with buttons that
add functions such as the ability to insert an image or a page break. In previous versions of
Joomla, this type of plug-in was called an editor-xtd.
❑ Search — Used to add search capabilities for a particular item type such as content, sections,
contacts, Web links, newsfeeds, or categories.
❑ System — Interfaces with the core Joomla system functions and allows control and modification
over foundation tasks such as publishing, unpublishing, and even installation.
❑ User—Used to synchronize a user database with another user privilege system such as Gallery2,
Simple Machines Forum, and so on.
❑ XML-RPC — Receives events directed through the XML Remote Procedure Call (RPC) communication
Here is a tutorial to create a content type plugin
For more about it visit developer Joomla! website