Resting between reps is a very important factor for people that want to improve their mucle strenght or size.I repeat strenght OR size.Its physically imposible to reach your peak in both body building and power lifting at the same time.

So assuming that you have already picked a direction im going to explain 1 by 1 about them.

First we will speak about body building.

People that want to increase their muscle size and have minimum amount of fat should rest 20-100 sec between sessions.

There is a experiment made with 4 groups of people.they did standard exercises with different amount of reps and rest:

Group 1-5 reps 1 min rest

Group 2-5 reps 3 min rest

Group 3-10 reps 1 min rest

Group 4-10 reps 3 min rest

The research showed that only group 3 had its level of growth hormone raised.So if you want big and beautiful muscles you have to cut the weight,cut the length of your rest and increase the amount of reps! For Power Lifters.People that want to increase their body strength have to make bigger rests and sets no longer than 30 secs.Your rest should be around 2-10 min.

One extreme example is Paul Anderson who took 30 min rests between his sets.The guy won the olympics in weight lifting,but also had a reasonable amount of fats so if you indeed want a clean and good looking body dont act like him.