It's called The X Window System, and it's the graphical user interface (GUI) for Linux. It's also known as X11 for short, or just plain X. Some call it X Windows, but for obvious reasons, this can annoy some folk.

Linux being a multi-user OS means pretty much every Linux component is grounded on the principal of client-server architecture.

This being said, you may have noticed an X server being installed on your computer during the installation of Linux. This X server controls the display of all those boxes and buttons your GUI uses to take form. There are many X servers available, each being tailored to a specific range of graphic cards. The installation program probes your card to determine the correct X server to install. The programs that use the X server to display themselves are referred to, as X clients.

The great thing about Linux is choice. And one of the greatest choices you have, is the insane assortment of ways you can customise the look, feel, and features of the GUI. Truly, the options at your disposal are staggering. Most of the thanks go to the person that first thought up the concept of a Window Manager (a special X client that tells the X server how to display the windows and interacts with the user). A feature that will have many a newbie, drooling with delight. :)

Changing from one Window Manager to another will radically change the look and layout of your GUI. Add to this the amount of customisation each Window Manager provides, and you have a recipe for some serious fun!

To spice things up (and complicating matters to boot), you also get GNOME, which works on top of a GNOME-compliant Window Manager ("Enlightment" being the default for Red Hat 6.0 and 6.1), on top of X! And all this running in its own shell - with the kernel at the core!!

And if you think that sounds complicated you get a piece of software called VMware which allows you to run Linux on top of NT (or vice-versa)!