What is circuit training?

In one line

Circuit training is a way of exercising in which you do several exercises in a set order with almost no rest, making a lap, and repeat the lap.

What is circuit training?
Going around a loop of exercises
In fitness classes or gyms you may see people moving through several exercises in a set order. This is called 'circuit training'. This post explains what circuit training is, how it differs from regular strength training, and what it changes in the body.
In short
Several exercises in a row, with almost no rest, making one lap
Circuit training means doing several different exercises in a set order, one after another, with little or no rest between them. Finishing the last exercise completes one lap, and you go around once or several times.
The name
A 'circuit' is a looped race track
A 'circuit' is a looped road used for car or motorcycle racing. Moving through the exercises in turn and coming back to the start looks like going around such a track. A Korean dictionary defines circuit training as an all-round fitness method that builds the muscles, breathing, and the body's blood circulation together.
For example
30 seconds per exercise, short rests
For example, you might do each planned exercise for 30 seconds and then go straight to the next one. Instead of time, each exercise can be set by count, such as 12 to 15 reps. Weights range from light to heavy depending on the purpose.
How it differs from regular strength training
Rotating instead of finishing one exercise first
A group of reps done without rest is called a 'set'. Regular strength training finishes all sets of one exercise before moving to the next, resting between sets. Circuit training does just one set of an exercise and then moves straight to the next, rotating through many exercises.
How it differs from supersets
A pair of exercises, or a loop of many
A superset pairs two exercises and does them back to back without rest. Circuit training links more exercises into one loop. Both cut rest time so that more exercise is packed into the same amount of time.
What the body does during it
A faster heartbeat and harder breathing
Because rests are short, the heart beats faster and the body uses more oxygen during circuit training than during regular strength training. People also rated it as harder. That is why it is also described as strength training combined with 'aerobic exercise', the kind that moves big muscles for a long time using oxygen.
What improves
Strength and cardio fitness together
In studies where people did circuit training regularly, strength in both the arms and legs went up. 'Cardio fitness', the ability to keep exercising using the body's big muscles, also improved. Both rose more than in people who did not exercise.
Goals it does not fit well
Strength for lifting the heaviest weight
The biggest strength gains came mostly from people who had not been exercising before. Because circuit training often uses fairly light weights and fatigue builds up, regular strength training is seen as the more direct way to build maximum lifting strength or the most muscle size.
Common misconception
Will circuit training make your weight drop fast?
Not for everyone. Body weight went down only in people who were heavy to begin with. People of normal weight showed no difference in body weight compared with people who did not exercise. Apart from body weight, across everyone who did circuit training, body fat went down and muscle went up a little.
Summary
Rotating through exercises with almost no rest
Circuit training means rotating through several exercises in a set order with almost no rest, and it builds strength and cardio fitness together. Body weight dropped only in people who were heavy to begin with. Related terms: superset, HIIT (high-intensity interval training), aerobic and anaerobic exercise.