A GUI or graphical user interface is a user interface based on graphics (icons and pictures and menus) instead of text; uses a mouse as well as a keyboard as an input device.  A GUI is a the system of menus and windows that allows users to interact with hardware and run software programs in a user-friendly visual environment. It also allows for easy navigation within the file structure. Some examples of different GUI's can be found below.  A GUI is often the way a user thinks of an operating system.  Linux's GUIs offer the user the ability to freely modify and customize the GUI to their own personal needs.  Windows XP's GUI gives the user only two choices of appearance and remains fairly generic.  Linux's GUI variety is one of its most rewarding characteristics and one of the most difficult aspects of the operating system for new users to adapt to.

KDE:

Gnome:

Xpde:

Windows XP's GUI: