A Graphical User Interface (GUI) is the major way in which user experience is affected because a GUI is the way a user communicates with the program. User interfaces are most diverse and some of the best known GUIs are Linux GUI, Windows GUI, etc. A good Graphical User Interface should follow usability rules.
Interface design is tricky but if you want to keep your customers happy, you should learn how to design interfaces in a professional manner. The resources in this section will certainly help you to learn more about Graphical User Interfaces and their principles. Study the articles and you will learn a lot about what good Graphical user Interfaces are and how to design them.