Plugins are bits of code that, once installed, extend the functionality of Movable Type. The word plugin is often used by software developers to describe a third party application that "hooks" into a main application to extend its functionality. Movable Type hosts an active plugin developer community whose plugins have greatly enhanced the capabilities of Movable Type. You can find a directory of MT plugins at http://www.sixapart.com/pronet/plugins/. To use a plugin, you will need to use an FTP program to load the plugin files to your server. When you upgrade your MT version, you must be careful not to overwrite your plugins file. Either that, or reload the plugins after the upgrade. Sometimes earlier MT version plugins will not work with the latest MT version. So make sure to check the documentation of the plugins before you install them to ensure compatibility.
Plugin Manager by Majordojo.