What is usability?
Usability is the degree to which something - software, hardware or anything else - is easy to use and a good fit for the people who use it.
It is a quality or characteristic of a product.
It is whether a product is efficient, effective and satisfying for those who use it.
Usability can be a rewarding as well as interesting profession. Many companies don't care for it and it effects their success. Anybody involved with software should read and study usability.
More about Usability can be found on:
http://www.upassoc.org/
To discuss about it you can join:
http://groups.yahoo.com/group/agile-usability-india/
"Usability is a term used to denote the ease with which people can employ a particular tool or other human-made object in order to achieve a particular goal. Usability can also refer to the methods of measuring usability and the study of the principles behind an object's perceived efficiency or elegance."
A more detailed description regarding this topic can be found at http://en.wikipedia.org/wiki/Usability