Usability is about effectiveness, efficiency and the overall satisfaction of the user.
Usability is refers to the quality of a user’s experience when interacting with products or systems and based on the following factors:
1. Intuitive design: a nearly effortless understanding of the architecture and navigation of the site
2. Ease of learning: how fast a user who has never seen the user interface before can accomplish basic tasks
3. Efficiency of use: How fast an experienced user can accomplish tasks
4. Memorability: after visiting the site, if a user can remember enough to use it effectively in future visits
5. Error frequency and severity: how often users make errors while using the system, how serious the errors are, and how users recover from the errors
6. Subjective satisfaction: If the user likes using the system
Reference: http://www.usability.gov/
