What is Web Usability?

Usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.

Effectiveness is the accuracy and completeness which specified users can achieve specified goals in particular environments.

Efficiency is the resources expended in relation to the accuracy and completeness of goals achieved.

Satisfaction is the comfort and acceptability of the work system to its users and other people affected by its use.

Therefore, "Web Usability" is the creation of Web sites that users want to use, with minimum amount of effort, time and frustration. Ultimately, you want each of your users to have the perfect experience on your site, or at least one which results in them getting what they want.