A group of consecutive repetitions of an exercise performed without resting is called a set.
For example, performing 10 squats continuously and then taking a break is one set of 10 repetitions. The individual movements are called repetitions, or reps; the set is the larger unit that contains them. A workout may prescribe several sets of an exercise, with rest periods between them.
Sets are a basic way to organize resistance training. Training volume is often described as sets × repetitions × load, although coaches may also account for exercise intensity, tempo, and proximity to muscular failure. Different goals commonly use different set-and-repetition schemes: very heavy strength work may involve only a few repetitions per set, while muscular endurance often uses many more.
An interval is a timed work-or-rest period and is not necessarily a group of repetitions. A circuit is a sequence of different exercises, usually performed in succession. A rep is one execution of the movement, not the entire uninterrupted group. In ordinary gym language, “three sets of eight” means eight repetitions, followed by rest, repeated three times.