Are you a Mac or ex-Mac user? I note from your input in another thread that you are dualbooting Windows 8.1 and Elementary, I hope you like the difference.
With "Desktop / X" - X is the graphical environment used by a majority of Linux Distros. It is provided by the folk at x.org , and is also know as xorg . From the moment you hit the login screen, and thence from the Desktop, you are in an X session.