What is the difference between a 'system plugin' and a 'user installed plugin'?

• System plugins are shipped with the application. These plugins are essential for the functioning of the system. Although you can disable some of them, you should not do so unless instructed by an Atlassian support engineer. Note: Not every system plugin can be disabled. You cannot uninstall any system plugins.
• User installed plugins are those which have been installed in the application after it was set up. You can install a plugin either by uploading a JAR file or via the Universal Plugin Manager. You can uninstall these plugins.