It's called circuit training because it often involves a series of exercises (usually 6 to 10) at different points or "stations" laid out in a circuit. The circuit will generally involve a series of resistance stations, using weights or body-weight exercises like push-ups and lunges. These exercises will target different muscle groups to strengthen and tone your whole body as you work your way around the course.
The resistance stations are interspersed with cardiovascular exercises, like shuttle runs and squat-thrusts, to strengthen your heart and lungs and, of course, to burn off that belly.
How does it work?
You can try circuit training on your own, but joining a circuit class is great for motivation and an instructor will make sure you're getting the most from your workout. Classes can take place indoors or outdoors and usually last between 45 minutes and an hour.
By the end of a class you might feel as though you've been through a marine-corps boot camp, but with regular circuit training you should see a rapid increase in your level of fitness - so you'll be a rippling GI Joe (or Jane) in no time.