Ubuntu on Wayland

at the graphical login screen, you may see an icon either next to the login name, or at the top of the screen on the right; the icon may look like a settings industrial toothed wheel icon, or it may look like the ubuntu 3-branch symbol;

if you click on this icon, you may see:

Ubuntu
Ubuntu on Wayland

or you may see this:

Ubuntu
Ubuntu on Xorg

Xorg and Wayland are display servers (not to be confused with display managers, such as lightdm or gdm3, or with desktop environments, such as KDE, Unity, LXDE, XFCE, Openbox, Fluxbox, etc)

what this means, seeing these 2 options, is that you have one desktop environment, namely Ubuntu, and 2 display servers installed, and you can choose whichever you want; so which one should you choose?

Xorg has been around a long time, so it is likely that all linux programs work with it; so if you don't want to have things not working anymore, choose Xorg;

Wayland is new, and some programs do not work with it, such as gparted, or screen recorders, etc; one question here even noted that Wayland turns off your nvidia, causing the laptop to heat up;

to see which display server you are using, in a terminal/at a command prompt try this:

echo $XDG_SESSION_TYPE

so, if you are trying Wayland and things don't work, log out, click on the icon on the login screen and choose Xorg then log back in.

Also if you have more than one display manager installed, for example you have gdm3 and lightdm, to check which one you are using, type at a terminal command prompt:

cat /etc/X11/default-display-manager

and to switch between them type

sudo dpkg-reconfigure lightdm

or

sudo dpkg-reconfigure gdm3

A text window appears where you can use the arrow keys to select one of the installed display managers and press enter.

It is possible to install other desktop environments as well, such as one or more of these or others, and then they will also appear in the list of desktop environments which you can choose from, before logging in:

sudo apt-get install xfce4
sudo apt-get install kde-plasma-desktop
sudo apt-get install lxde
sudo apt-get install openbox
sudo apt-get install fluxbox

Then the desktop environments list to choose from before login might look like this:

Fluxbox
LXDE
Openbox
Plasma
Ubuntu
Ubuntu on Wayland
Xfce Session