A rep is the completion of an exercise, while a set is composed of one or more reps, or repetitions. A rep is one sit-up, one lunge or one arm curl. A set is a series of reps, performed continuously, ending with a rest period.
If a person were to perform sit-ups in three sets of 10 reps, they would be completing a total of 30 sit-ups. The first set would consist of 10 sit-ups, followed by a rest period. Then the second set would start, consisting of 10 sit-ups, and followed by a rest period. Finally, the third set would consist of 10 sit-ups, completing the three sets.
The concept of sets and reps applies more to weight training, which helps trainers to ease their way into increasingly more challenging regimens as they build muscle. Lower numbers of reps allows trainers to lift more weight and build muscle faster. Higher numbers of reps usually require lighter weights and help tone muscle.