One very important variable of a fitness program, and one that's often overlooked, is rest periods.

What a rest period ultimately comes down to is either:
1. Performance
2. Heart rate/calorie burn

In the most basic sense, a shorter rest period will lend itself to a higher heart rate for a longer period of time. In turn, the higher your heart rate and the longer this elevated heart rate is sustained, the more calories you will burn. So, a shorter rest period will contribute to a bigger calorie burn in a smaller amount of time. Rest less, burn more!

Conversely, if you are training for performance, you'll want to approach your rest periods in almost the exact opposite way. You'll want to rest the MAXIMUM amount of time within an optimal window of 3-5 minutes in order to increase performance. The longer you rest within that optimal window, the better your performance will be, and vice versa. The bigger the rest, the bigger the lift!

So, this also means that shorter rest periods will contribute to lower performance than a longer rest period would. Because of this decreased performance with shorter periods of rest, a program utilizing them will be working with lighter weights (or maybe no weights at all) than the same workout with longer rests. For example:

Let's say I'm going to do squats. If I want to do them for performance my workout might look like this:
- 1-5 warm up sets
- #210 lbs x 5 reps
3-5 minutes of rest
- #210 lbs x 5 reps
3-5 minutes of rest
- and so on until I'm done

But, if I wanted to do a weight training workout for squats with shorter rest periods, instead of doing #210 for 5 reps with longer rests, it may look something like this:
- 1-5 warm up sets
- #155 lbs x 10 reps
60-90 seconds of rest
- #155 lbs x 10 reps
60-90 seconds of rest
- and so on...

(You'll also notice that the amount of reps changed from 5 to 10 when the goals and rest periods changed. Check out Part 3 of this series for more details about how to choose your set/rep schemes.)

OR, for an example of something totally different, here's a squat workout with minimal rest that uses no weights at all:
Warm up
- 30 squat pulses
- 20 air squats
- 10 jump squats
- 5 box jumps
x 5 rounds
no rest between exercises
30-60s rest between rounds

So what do these different styles of resting achieve? In general, but not always, longer rest periods for the purpose of increasing performance will accompany strength gain goals. (Think: powerlifting or olympic weightlifting.) And, in general, shorter rest periods for the purpose of increasing calorie burn will work towards weight loss goals or the desire to alter body composition. (Think: HIIT training or body-building style training.)

One last thing to touch on: that optimal window of 3-5 minutes for a longer rest interval. How did those numbers get chosen? Well, when you perform a high intensity set of an exercise your body has to produce and use the fuel within your muscles to complete the activity. This results in a depletion of said fuel by the end of the set. In order to perform again at your highest possible capacity for the next set, you want as much of that fuel to be replenished as possible. Through sports research studies it's been shown that the optimal window of time for replenishment is between 3 and 5 minutes. Obviously, the harder you work, the longer you'll want to rest, and vice versa. Lastly, there is no magic switch that flips right at the 5 minute mark that cuts off this replenishment, so when your previous set was intense enough, it may be necessary to rest even longer than 5 minutes. There have been times when I've rested for up to 7 or 8 before moving on. (Quick tip: use a stopwatch or a timer, so you don't completely lose track of how long you're resting.) 
Sources:…/manipulating_resistance_training_… , ,…/effects_of_different_weight_train… , (also supported by NASM textbook 4th edition, and Mark Rippetoe's Practical Programming 3rd edition).