Usability testing is a usability practice intended to evaluate a product or information product by testing it with users against a pre-defined set of criteria. Usability testing involves the following activities:
The STC Usability and User Experience Special Interest Group has a usability toolkit of forms and other materials to help you plan and conduct your test.
Dana Chisnell, STC Fellow and co-author of "Handbook of Usability Testing" (Rubin and Chisnell, 2008, Wiley) blogs about usability testing. Particularly useful posts:
Two textbooks by STC Fellows, both with lots of practical advice:
A short, lively book in Steve Krug's inimitable style, aimed at people who want a quick way to conduct their first usability test: