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.