“Most graphical user interfaces are all-in-one systems.
Often, with Windows and MacOS for example, they are integrated
right into the operating system. This makes things nice and
consistent for the user, but this consistency comes at the expense
of choice.
Linux, like most forms of Unix, takes a different
approach. It uses the X Window System. The X Window System
(also called X11, or simply X) de-couples the graphics system from
the OS, and splits it into two components: One component, called
the X server, draws the dots and lines on your monitor; a second
component, called the client, tells the server what to draw and
keeps track of what’s going on within the various windows on your
desktop. In fact, these two components can even reside on different
computers and communicate across a network. The X server has to run
on your desktop, but it can be controlled by a client a continent
away, if you choose.”
“The window manager is another important part of the X Window
System. The window manager is a client to the X server that runs
alongside other clients, handling a lot of the management work of
manipulating and displaying the windows on your desktop. You can
run only one window manager at a time, and they enjoy special
privileges. For example, when an application requests a new window
on your screen, the X server won’t create it until the window
manager says where it should be placed.”