Interval training is exercises or workouts that use short high intensity periods separated by low intensity exercise. The purpose is to make the heart rate go up for a period of time and then back down, just to go back up again. When the body is under that type of stress it tends to use fat as its primary source of energy. It is also a very effective at keeping your metabolism high and burning calories long after you are done exercising. One basic way of doing intervals is by running sprints with short breaks in between.