user interface design or user interface engineering is the design of computers, appliances, machines, mobile communication devices, software applications, and websites with the focus on the user's experience and interaction. The goal of user interface design is to make the user's interaction as simple and efficient as possible, in terms of accomplishing user goals—what is often called user-centered design.
Reference: Wikipedia http://en.wikipedia.org/wiki/GUI_design, retreived 2011-04-25
While GUI-design is based on reasearch, information architecture, prototypes, usability considerations, it referes to the design of the graphical user interface, i.e. the look and feel of the actual implementation.