When you start a desktop session for the first time, you should see a default startup screen, with panels, windows, and various icons.

The major components of the GNOME Desktop are as follows:

Desktop
The desktop itself is behind all of the other components on the desktop. You can place objects on the desktop to access your files and directories quickly, or to start applications that you use often. 

Panels
The panels are the two bars that run along the top and bottom of the screen. By default, the top panel shows you the GNOME main menu bar, the date and time, and a set of application launcher icons, and the bottom panel shows you the list of open windows and the workspace switcher. Panels can be customized to contain a variety of tools, such as other menus and launchers, and small utility applications, called
panel applets. For example, you can configure your panel to display the current weather for your location.

Windows
Most applications run inside of one or more windows. You can display multiple windows on your desktop at the same time. Windows can be resized and moved around to accommodate your workflow.
Each window has a titlebar at the top with buttons which allow you to minimize, maximize, and close the window.

Workspaces
You can subdivide your desktop into separate workspaces.  Each workspace can contain
several windows, allowing you to group related tasks together.

File Manager
The Nautilus file manager provides access to your files, folders, and applications. You can manage the contents of folders in the file manager and open the files in the appropriate applications.

System Menu
You can customize your computer using the System Menu. This contains the Preferences menu (which allows each user to customise their desktop) and the Administration menu (which allows computer administrators to customise the settings for all users). The System Menu also contains launchers for this help system and items for locking, logging out from and shutting down your computer.

Your vendor or system administrator can make configuration changes to suit your needs, so your desktop might not match exactly what is described on this website. Nevertheless, this website provides a useful introduction to using the various components of your desktop.