UsabilityUsability Testing
Usability testing is a big subject but can be divided into 2 areas: expert review and user testing.
Expert Review
Involves the review of a site by a usability expert who can uses his/her experience to identify usability issues with a site. Sometimes refered to as Heuristic evaluation this is a technique championed by Jakob Nielsen and is a method for finding usability problems in a user interface design and is often used as part of an iterative design process. Heuristic evaluation involves examining a web site against a recognized set of usability principles or checklist (the "heuristics").
The advantage of expert review is greater speed and often less cost compared to user testing.
User Testing
User testing utilises test users (subjects) who are observed completing set tasks on a website. Recruiting even a small number of subjects representative of actual users can be challenging but worthwhile as users will identify issues that the develop team will not have been aware of. Test subjects are given a scenarios to work through and are asked to "think aloud" as they use the site and encounter usability problems. Valuable insights can even be gained from running quick user tests with people from around the office unconnected with development of the interface. But if the budget exists a usability expert will be able to write suitable descriptions of users (subject briefs) to allow recruitment of appropriate subjects and write precise test scenarios that will enable you to get the most out of your testing. An expert may also be able to complete an initial review of your site's usability prior to testing to enable majpor issues to be ironed out beforehand.
Reading List
Designing Web Usability: The Practice of Simplicity by Jakob Nielsen
Don't Make Me Think!: A Common Sense Approach to Web Usability by Steve Krug
Also take a look at the following companies Uservision and Bunnyfoot.