They are desktops. The graphical environment you interact with. Windows and file browser and save dialogs and settings screens.
Unlike Windows and Mac there is no single default desktop. There are a number of them. The most popular ones are Gnome (default on Ubuntu) and KDE. They're polished, general purpose and work well.
Other desktops, such as Cinnamon used by mint, tend to be more niche or special purpose.
