"Usability is important if you want to ensure developing a good quality product (at least in terms of interaction), and it directly affects how your users feel about your product (the user experience), both of which should be part of the goal of every self-respecting designer."
"Usability is a measurement of how well a system allows us to interact with it and achieve our goals. It focuses on users, systems, environments and tasks."
"Usability is NOT to be confused with User Experience. While usability deals with how we interact with a product, user experience deals with how we FEEL about this interaction. It is an (emotional) assessment of how we perceive a product and our interaction with it, whether we are satisfied and happy or frustrated and disillusioned."
The real importance of usability and user experience