If you are using a Linux operating system as you read this, lean back and take a look at the desktop. Chances are you've got a few things running; a browser, an instant messaging client, and maybe even a twitter client. You might be listening to music in any number of apps, and chances are you are running all of them in either the Gnome or KDE window manager, with Compiz providing a layer of 3D on top of it. Supporting the window manager is X, the graphics engine that has been powering graphics on Unix systems for the past thirty years. If you dig farther down, you'll find a shell running, and under that, way down deep, you'll find the kernel. This kernel is Linux, and it gives access to the hardware to all of the software built on top of it.