Q. How do I start or stop X.Org from a command prompt?

A. X.org is open-source implementation of the X11 system. It is an open source X11-based desktop infrastructure. Xorg provides an interface between your hardware and the graphical software you want to run. Besides that, Xorg is also fully network-aware, meaning you are able to run an application on one system while viewing it on a different one.

Start X.Org Server

There are two ways, just type startx:
$ startx
Alternatively, run init script such as /etc/init.d/gdm
# /etc/init.d/gdm start
OR
$ sudo /etc/init.d/gdm start

Ctrl+Alt+Backspace – to stop server

Press Ctrl+Alt+Backspace to immediately kill the server – no questions asked.

Agian, you can use init script to stop session:
# /etc/init.d/gdm stop
OR
$ sudo /etc/init.d/gdm stop