In the log in screen there is a small settings icons beside 'sign in'. There are some options in there including 'ubuntu', 'gnome' etc. I am familliar with these but what's the significance of choosing one of them there?
Usually the options are for choosing between types of desktop environment and display servers.
If you have more than one desktop environment installed on your system, you would be able to choose one of them. Some examples for desktop environments are: Gnome, Cinnamon, Unity,Xfce, KDE and MATE.Main difference between desktop environments are the look of them and the amount of resources(RAM) they use.
if you have more than one display server installed on your system, you would be able to choose one of them. Some examples for display servers are Wayland and Xorg. Some programs may have problem running on some display servers. And also Ubuntu is the name of GNU/Linux distribution you are using.