These are the factors that will determine whether or not your site is a success. If your site is difficult to use, not forgiving of user errors, and information is not readily available, the experience of the user will be less than satisfactory. We want people to “feel good” about using your site or application, and come back soon. Knowing that you can achieve outcomes is most important to the majority of users.
[Usability refers to] 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.—ISO 9241 – 11