What's the difference between "reps" and "sets"?
"Reps" stands for "repetitions." It's literally how many times in a row you're going to do an exercise. For example, if you do 10 reps of lat pulldowns, as I did today at the gym, you pull the bar down toward your chest 10 times in a row.
Then "sets" refers to how many groups of those repetitions you're doing. So "3 sets of 10" means you're doing 10 repetitions in a row, taking a break, doing another 10, taking another break, and doing a final 10.
In short, a rep is one execution of any exercise, whereas a set is a combination of any number of reps of one exercise.