The premise of interval training is simple:

The premise of interval training is simple: When you vary your effort by mixing periods of high and low intensities during your workout, your fitness will improve faster and more dramatically—and your workouts will be less boring. During your session, you’ll alternate between shorter, high-intensity intervals and longer, lower-intensity recovery periods. The high-intensity intervals can be "anaerobic" (where you are working as hard as you can, and your heart rate is usually over 85% of your estimated maximum), or simply more intense, like in the 75-85% range, which is still “aerobic.”

You'll know when you’ve reach an anaerobic intensity because you'll start feeling a burn in your working muscles. Adding some anaerobic intervals to your workouts will usually give you better results. But since they are more demanding, anaerobic intervals should be shorter and accompanied by longer recovery intervals. As your fitnesslevel improves, both the length of the high-intensity intervals, and the amount of work you can handle during them, will increase.
During a complete workout, you go through five to ten cycles of high and low intensity. Depending on your fitnessneeds, you can vary the length of each interval, number of intervals, distance, and speed.