Cycle through several exercises (usually five to 10) targeting different muscle groups with minimal rest in between.
The result is a workout that taxes your muscular strength and endurance and your cardiorespiratory system. You've likely done circuit training in boot camp-style classes and didn't even realize it. Circuit training workouts are based around a set number of "stations" that you repeat until your time runs out. So knowing how much time you have can help you determine how many circuits you'll need to complete and how hard you'll need to work.