Usability refers to the effectiveness and efficiency of a system in meeting the goals and expectations of its users. All complex systems raise usability issues. Usability is not a single trait of a system but rather an umbrella term encompassing a number of distinct (and often conflicting) traits, including learnability, efficiency, effectiveness, and satisfaction. Moreover, these traits are not intrinsic to the system but must each be evaluated with respect to specific classes of users. For example, what is intuitive and therefore effective for a casual or beginning user may be tedious and verbose to an experienced user.