Cinnamon is a Desktop Environment (DE)... like the others mentioned before: GNOME, XFCE, KDE, LXQT, and MATE are all Desktop Environments. And there are others. Besides DE's there are also Window Managers (WM), such as Fluxbox, i3, IceWM, JWM (Joe's Window Manager), and others. Each product is a labor of love by those who make them to share their vision of how things should look and how they should work. But everyone has different ideas, so you have choices.
WM's and DE's are what gives you the look and feel of the graphical desktop.... while all are using the Linux kernel under the hood (not always the same kernel). Window Managers are typically "lighter" (need less RAM and CPU resources) than Desktop Environments, so they may be preferred on very old computers. There are some differences in behavior too, and it's good to test some samples of both as you learn your way around.
XFCE, KDE and the original GNOME are "old timers" of the X Desktop world, but others branched off from them, and GNOME has reinvented itself over the years.
MATE is a fork of GNOME 2, and
Cinnamon is a fork of GNOME 3.
Both DE's and WM's give various options and controls for the user to change each one to suit their own needs. But they can't all do everything, nor suit everyone. You can download or create your own themes to customize your desktop. There are many applets that can run on the desktop, like local weather or system usage (memory, hard disk space, etc).