The authentication plugins operate when users login to your site or administrator. The Joomla! authentication is in operation by default but you can enable Gmail or LDAP or install a plugin for a different system. An example is included that may be used to create a new authentication plugin.
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.
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.