A Usability lab is a place where Usability testing is done. It is an environment where users are studied interacting with a system for the sake of evaluating the system's usability.
Depending on the kind of system that is evaluated, the user sits in front of a personal computer or stands in front of the systems interface, alongside a facilitator who gives the user tasks to perform. Behind a one-way mirror, a number of observers watch the interaction, make notes, and ensure the activity is recorded. Very often the testing and the observing room are not placed alongside. In this case the video and audio observation are transmitted through a (wireless) network and broadcasted via a video monitor or video beamer and loudspeakers. Usually, sessions will be filmed and the software will log interaction details.