How do people interact with the web site you are testing? What is cumbersome or simple for people to do? What is logical and rational? What is dynamic? What would users prefer to see? What do they dislike about it? The way users interact with your web site may reveal bugs that you cannot see. They may also make recommendations that you did not originally foresee as a need.
User interface design determines the level of effort required by the user to function within the web site and to define the displayed content of the web site, in addition to the effort required to figure out how to accomplish the task. Usability is understanding and applying human needs to a specific user interface to expedite the ease-of-use of the web site while satisfying the user objectives.
Usability levels are defined by the performance rating of an interactive experience when applied to user objectives within a particular context. Of course, you ideally want your users to find what they want when they want it and as simply and quickly as humanly possible (for the average human).
