Usability refers to how usable a product is by the intended user.
Usability is defined by five
There are many other important quality attributes. A key one is utility, which refers to the design's functionality: Does it do what users need? Usability and utility are equally important: It matters little that something is easy if it's not what you want. It's also no good if the system can hypothetically do what you want, but you can't make it happen because the user interface is too difficult. To study a design's utility, you can use the same user research methods that improve usability.
User testing should be done during product development so results from the testing can be incorporated into the product. Depending on the stage of the design process and the aspect of usability you want to focus on there are different techniques
· Usability testing
o Guide to Planning and Conducting Usability Testing (from University of Washington Libraries) http://www.lib.washington.edu/usability/guidelines/usability-guidelines.html
o Seven Common Usability Testing Mistakes http://www.uie.com/articles/usability_testing_mistakes/
· Focus groups
· Card sorting