Usability is measure of quality of a user's experience when interacting with a product or system - whether a web site, software application, mobile technology, or any user-operated device.
According to Usability Expert Jakob Nielsen: "On Web, usability is a necessary condition for survival. If a web site is difficult to use, people leave. If homepage fails to clearly state what a company offers and what users can do on site, people leave. If users get lost on a web site, they leave. If a web site's information is hard to read or doesn't answer users' key questions, they leave. Note a pattern here? There's no such thing as a user reading a web site manual or otherwise spending much time trying to figure out an interface. There are plenty of other web sites available; leaving is first line of defense when users encounter a difficulty." More: http://www.useit.com/alertbox/20030825.html
What is usability
Is your web site usable? There are several definitions for usability, but basically following 4 characteristics can be considered usable:
* Quick and easy to learn; * Efficient to use; * Allows rapid recovery from errors; * Easy to remember;
Does your product or web site have above characteristics? Usability increases benefits for both parties: User (potential customers) and Provider (yourself).
Users benefits from usability * Users are satisfied, not frustrated, with product or web site; * They enjoy interacting with web site or product; * They achieve their goals effectively and efficiently; * They cultivate confidence and trust in product or web site.