A repetition is doing the movement 1 time (for example, 1 push-up). Most of
the time, you will be instructed to do a given number of repetitions (such as
15). This means you are to do the same movement 15 times (15 push-ups) without
resting in between.
A set is the completion of a series of repetitions. Once you've completed 15
push-ups, you're done with 1 set. If you are instructed to do 2 sets, you
may rest in between them for a minute or so (if you are sore, you may want
to stretch your muscles).
Many people will work 2 muscle groups at the same time by completing 1 set of bicep curls immediately followed by 1 set of tricep kickbacks. This method is called "super setting". If you want to use this method, make sure you work opposing muscle groups (such as bicep/tricep, quadricep/hamstrings) in order to keep the blood flow to the same region of the body. When using this method, you will not take any rest time in between sets because the muscle will get a break when you are working the opposing muscle group.