Hello, UX!
Usability testing is a particular sub-category of user testing that asks users to perform tasks on a prototype of a site or project. The user is then asked questions to gauge whether they are interacting with the design as expected, if unexpected preferences arise, or if there are particular problems or bottlenecks that need addressing.
For more, see the Nielsen Norman Group's resources on usability testing 101.
Below, I have linked a rough questionnaire that I've used to assess the usability of a beta version of a website. The test was performed with a class of students; it was given as a homework assignment, so the test was not conducted with a facilitator. This isn't optimal - in a best case scenario, there would be a facilitator to observe and prompt the user - but in a pinch it's a method that can be used to get at least some information on how the site is working.