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.
The excerpt is the ISO definition of usability.
In terms of a web site, the aim of usability is to make that web site easy to use whilst the aim of user experience is to make the user happy before, during and after using that web site. Thus, usability relates to the ease with which users can achieve their goals while interacting with a web site while user experience is concerned with the way users perceive their interaction with that web site.
Usability consultant Jakob Nielsen and computer science professor Ben Shneiderman have written (separately) about a framework of system acceptability, where usability is a part of "usefulness" and is composed of:
- Learnability: How easy is it for users to accomplish basic tasks the first time they encounter the design?
- Efficiency: Once users have learned the design, how quickly can they perform tasks?
- Memorability: When users return to the design after a period of not using it, how easily can they re establish proficiency?
- Errors: How many errors do users make, how severe are these errors, and how easily can they recover from the errors?
- Satisfaction: How pleasant is it to use the design?