From bemoko developer wiki

Introduction

Usability testing allows you to evaluate a site by testing on real users and seeing whether the site meets the intended purpose. It should also highlight room for improvements. Usability testing should be a continuous process of evaluation and refinement which should start early on in the development process, for example during story board creation, and continue through the life cycle of deployed site, especially during updates.

Further Reading

• Overview of usability testing - http://en.wikipedia.org/wiki/Usability_testing
• Lessons from Taptu's user experience team - http://blog.taptu.com/2007/12/04/hi-from-the-user-experience-team/