Ubuntu comes in versions with different desktops. The default version directly produced by Canonical, the company behind Ubuntu, uses a modified Gnome Shell desktop. Other versions exist that are officially recognized and supported by Cononical. Kubuntu comes with the KDE Plasma desktop, Xubuntu with the xfce desktop, Ubuntu Budgie with the Budgie desktop, Lubuntu with the lxde desktop, etc.
In principle, you can install additional desktop environments on a single Ubuntu installation. This is usefull if you have different users insisting on using a different desktop environment. It is not ideal, however, becaue it will lead to some clutter and sometimes minor issues and (mostly visual) inconsistencies. Especially software menus become cluttered, because all applications from all desktop environments appear in all menus.
If you are the sole user of the system, and want to switch definitively to another desktop, a clean reinstall is certainly preferred, as it will fully give you the seamless experience intended by the team of that particular Ubuntu version.
With respect to the terminal and core command line tools: these remains unchanged. The differences are only in the graphical user environment and the applications that are shipped with them.
