There are lots of shapes and styles of workouts…weight lifting, powerlifting, bodybuilding, Olympic lifting, power training, and Cross-fit; just a few.
The circuit idea has much in common with Interval Training which has been around a long time and is the basis of much athletic training. Its key principle is to vary a workout or training session between periods of intense effort and rest periods – either passive (pure) rests, or active rests, where the trainer proceeds at a much lower pace. The result is you get more intense training done in a given period, and more done overall towards improved performance.
In the Interval Training model, exercise intervals and recovery periods can range from a few seconds to several minutes. For example, a short, more intense work interval generally requires greater anaerobic metabolism and the recruitment of muscle fibres that can shorten the fastest, (fast-twitch) fibres which use the anaerobic energy system, while longer, moderately paced intervals typically invoke greater use of the aerobic system. (Anaerobic + aerobic briefly described here)
If you'd like to get into circuits with variable strength-training and aerobic achievement, take a look at this: Do It All With Circuits