Internet users spend extremly short periods of time on trying to use a web site.
If they don t find what they are searching for than within seconds they leave the site. Only satisfied users will use that site and come back. Usability means quality of interaction. Product developers more and more understand, that the user must play the most important role in the development of a system. Not the user should have to adapt to a system but the system should be designed to fit the needs of the user.