http://www.intervaltraining.net/index.html

Interval training is basically exercise which consists of activity at high intensity for 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 High Intensity Interval Training 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.

http://www.cptips.com/intervl.htm

http://findarticles.com/p/articles/mi_m1608/is_3_21/ai_n13557373/

http://www.ultracycling.com/training/intensity_training.html

http://www.trifuel.com/triathlon/bike/interval-training-the-scientific-way-000885.php