System – Access to the control panel, site settings, pending or locked content items, the option to clear cache and view system information (site, server, and environment).
Users – Manage users, user groups and access levels, add notes about users, deal with privacy related requests, view user logs and mass email everyone on your site.
Menus – As the name suggests, this contains all options about the creation and management of menus.
Content – Add articles and taxonomies, assign featured content and manage your site’s media files.
Components – Create and manage site banners, contacts, site updates, private messages, multilingual associations, and news feeds. It also provides access to post-installation messages (we already covered those), lets you set up redirects (though you need a plugin for that), view site search terms, use the smart search (again, this needs a plugin) and manage tags.
Extensions – Allows you to install, update, manage, configure, find and troubleshoot extensions. In this menu, you can also see the state of your database and update your site.
Help – Direct access to important help topics in the official documentation.