Graphical User Interface development for applications or web sites. This tag should usually be combined with other tags specifying the programming language, platform or framework, if applicable.

A Graphical User Interface is any kind of visual interface which is capable of displaying more than just fixed-width text. GUIs can display information in the form of images and other abstract graphics, and present interactive controls such as buttons, menus, icons and windows.

There's now a Stack Exchange site devoted to user interface design: