Usability testing is a hands-on, interactive assessment that helps you identify design flaws in your website that could be making it difficult for your users to find or do what they need.
Usability testing is not intended to help you find out how users feel about your website.
How to Conduct a Usability Test
Basic Idea: create a set of typical tasks and then observe your users as they try to perform those tasks.
Step by Step
Taking Action on Results
Screen Capture Tools
Mobile App Testing
Because we don't have any easy applications for captursing mobile app screens, the best recommendation I've seen is to use a document camera, such as an Elmo. Here's an article from the Nielsen Norman Group talking about setting it up.