What is the difference between a circuit and a set?

A set is the number of reps that you complete in one cycle. So if an exercise tells you to do 3 sets of 10 reps. It means you perform 10 repetitions of the exercise, then pause. Those 10 repetitions are one set. Then repeat for 2 more sets for a total of 3 sets.

A circuit is a grouping of two or more exercises that are done back to back. An example of a circuit repeated two times could be exercise A for 10 reps, exercise B for 15 reps. This would mean you would perform exercise A for 10 repetitions, then exercise B for 15 repetitions, then repeat exercise A for 10 repetitions, then exercise B for 10 repetitions.