What is the X server?

X is an application that manages one or more graphics displays and one or more input devices (keyboard, mouse, etc.) connected to the computer.

It works as a server and can run on the local computer or on another computer on the network. Services can communicate with the X server to display graphical interfaces and receive input from the user.

It's worth noting, a common component used with an X server is the Window Manager, an application that manages the resizing and moving of windows and decorative elements of windows such as title bars, minimize, and close buttons.

The X server can be started with the 'startx' command, or more commonly, from a display manager such as gdm.

~/.xinitrc is a shell script used by xinit, that starts the X server when not using a display manager, to define some application to start automatically in the X server.

/etc/X11/xorg.conf is a configuration file used to give the X server information about the hardware components used, but now the X server can avoid using it, because it is capable of autoconfiguring itself.

As the above poster mentioned, X is a server (meaning a program which other programs call upon and be called by) which is responsible for creating a graphical environment and if it fails for whatever reason, you'll be greeted by Command Line Interface (CLI).

The term server can also be said to apply to PulseAudio, which is the sound server, and it calls applications and is called upon to produce sound.