What is Interval Training?

Interval training is basically exercise which consists of activity at high intensityfor a period of time, followed by low intensity exercise for a period of time. These 'sets' are repeated.

Sprint Intervals

The high intensity portion are called Sprint Intervals. Sprint intervals are measured either by time or distance. They can be as short as 15 seconds in activities like HIIT or as long as 20 minutes for aerobic interval training.

An example of a Sprint Intervals would be running at full pace along a stretch of field for 30 seconds, another would be an indoor cyclist spending 15 minutes simulating a climb on the bike.

Rest Intervals

The periods of recovery are called Rest Intervals. During an rest interval athletes do not stop the activity but generally exercise at a low intensity which allows the body to recover from the sprint interval. The length of these rest intervals are determined primarily by your fitness levels and the type of the sprint interval.
The intervals are important; the basis of the interval training is to ensure that your sprints are done at an optimal intensity, without sufficient rest your interval training will resort back to an aerobic type of activity.

Intensity

The intensity of the sprint intervals is how hard you push yourself during the sprint. For simplicity sake the intensity is usually measured on a scale of 1 to 10, 1 being no effort whatsoever while 10 represents the maximum effort possible.
Now this is a completely personal scale depending on your own fitness levels and the type of interval training.
For example a seasoned athletes may be training on improving his speed work, the sprint lasts for 15 seconds and his ‘10’ may be a flat out sprint with the athlete going as fast as he or she can.
Another example would be someone who has not exercised in a while decides to do intervals while walking; a 1 minute walk at a brisk pace may leave the person completely out of breath. This would be their 10.
Now take the original athlete who sprinted for 15 seconds and change the interval to 5 minutes. They couldn’t maintain the same original 15 seconds pace for 5 minutes, but the maximum effort still represents a 10.
A ‘10’ is merely the maximum amount of effort a person can safely expend for that particular interval.

Article found on  http://www.intervaltraining.net/what_is_interval_training.html