A Brief Tour of the Joomla Administration Interface
If you're dealing with Joomla for the first time, take 20 minutes to look through the important admin screens.
System > Global Configuration
Through this interface you can manipulate the contents of Joomla's configuration.php file which sits at the root of your Joomla site. You can configure the settings for your database, email, debugging, site title tags, offline mode, default editor, base permissions, etc.
Users
Click on Users > Manage so you are here:
/administrator/index.php?option=com_users&view=users
Notice you have an options button in the top right and under the manage users menu item you've got Groups and Access Levels. Fields and notes are entirely options extras, but groups and access levels are always important.
Joomla has always been built with sophisticated Access Control out of the box. The basic idea is that a user can be placed into any number of usergroups, and usergroups can be assigned any number of access levels. Then you can set the action permissions of a component (like edit and delete) to map to an access level and you can set the viewing access of any menu item, article, component, or module to map to an access level too. There are a bunch of groups and access levels by default and frequently they will be all you need.
Menus
In Joomla the menu is in charge of the first round of SEF routing. If a menu item is pointed at a component, then that component can also employ its own router starting from that menu item.
Do note that the non-SEF routing will continue to work even when you have SEF routing turned on. So, for example, if you set a menu item to registered but have left the article public, then the article is still accessible through the non-SEF uri.
Content
The com_categories and com_content components are very commonly used but are still just components like any other and thus you can run a Joomla site without ever touching either.
Components
Not every component has a backend interface, but if it does then it's probably here. Of special note is the Joomla upgrade component. Yep, this is the component which checks for new Joomla versions, downloads the patch files, and runs the upgrade.
Extensions
Under Manage you can see two ways to install extensions. If it's a distributed extension then it'll be a zip file. The xml manifest at the root of the zip file will take care of what's being installed or upgraded etc. If you're installing your own custom extensions then you don't need to bother with zipping up your files. Just stick them right into place and visit 'discover' here:
/administrator/index.php?option=com_installer&view=discover
Click the discover button and it'll search for manifests in the right spot that don't have an entry in the extensions table. Once found just select your extension and install. Most of the time you just need the folder to have the right name and the manifest to be constructed properly. You can keep adding folders and files after you've installed.