The user interface is one of the most important parts of any program because it determines how easily you can make the program do what you want. A powerful program with a poorly designed user interface has little value. Graphical user interfaces (GUIs) that use windows, icons, and pop-up menus have become standard on personal computers.

Yahoo!'s user interface page
Yahoo!'s directory of user interface.

Maintaining a Responsive UI
A frozen user interface is hard to distinguish from a hung application. Learn how to address this problem in your .NET Windows Forms applications.