X window is a standard GUI on Unix for over 20 years. Implementation on linux organized by the XFree86 project.
From the user side, the concept of the actual interface is very different from the concept of Microsoft Windows.
X window only provides a graphical environment, not the actual graphics display to the user. Graphic display will depend on special software called a Window Manager.
Window Manager for X which is still very simple first is TWM. Window Manager of the ever popular, especially in academic circles is FVWM, due to its stability.
This initial interface priority compared to the ease of functionality.
Since 1996, the trend changed, starting with modifications re-FVWM. The main change is the user take advantage of the theme. The first is the Enlightment Window Manager.
His ability to be configured is the next important step, for example on BlackBox.
The next revolution is like the KDE desktop environment.