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