Users of web documents don’t just look at information, they interact with it in novel ways that have no precedents in paper document design; therefore, web designers must be versed in the art and science of interface design. The graphic user interface (gui) of a computer system comprises the interaction metaphors, images, and concepts used to convey function and meaning on the computer screen. It also includes the detailed visual characteristics of every component of the graphic interface and the functional sequence of interactions over time that produce the characteristic look and feel of web pages.
Source: WebStyle Guide, 3rd Edition: http://webstyleguide.com/wsg3/4-interface-design/index.html
Usability is a measure of effectiveness. It describes how effective tools and information sources are in helping us accomplish tasks. The more usable the tool, the better we are able to achieve our goals. Many tools help us overcome physical limitations by making us stronger, faster, and more sharp-sighted. But tools can be frustrating or even disabling. When we encounter a tool that we cannot work with, either because it is poorly designed or because its design does not take into account our needs, we are limited in what we can accomplish.
Web Style Guide: Basic Design Principles for Creating Web Sites, by Patrick J. Lynch and Sarah Horton: http://webstyleguide.com/index.html