Xorg

Xorg is an open source implementation of the X Window System (frequenty abbreviated to X or X11).

Xorg provides an interface between hardware and the graphical software which you want to run.

1. X session The combination of an X server and connecting X clients.
2. Display Managers Provide X users with a graphical login prompt

X uses a client server model. A given X server runs on one user's local machine, whilst the client programs which communicate with the server may run on different remote machines. This is the reverse of the common configuration of client-server systems, where the client runs on the user's computer and the server runs on a remote computer.

For example I can run emacs on my local machine, and connect via ssh to a remote CS server at uni to run say libreoffice, which would be running on the remote server so I can access my school files, but the libre office window would be rendered on my local machine, and it would also process my keyboard input (assuming of course the SSH session is configured correctly).

An X session is usually started by either the startx command or a daemon X display manager daemon program started by systemd based on the dependency of graphical.target.

1. Running startx basically runs xinit (startx is a user-friendly wrapper).
2. startx first looks for ~/.xinitrc which, unless there is a very good reason, should not be on a Debian system (use .xsession instead - see the startx manpage for why you should do this).
3. startx now searches for the system xinitrc in etc/X11/xinit. This contains the line ". etc/X11/Xsession" and Xsession will use ~.xsession if it exists.

See this stackexchange post for more details.