Interval training is an important ingredient in increasing athletic performance. It is recommended for swimmers, runners, skaters, and cyclists, and I assume it is valuable to most other athletes. Interval training consists of short periods of intense training separated by short recovery periods.

These bursts of activity, at full exertion, force your body to utilize its anaerobic system to produce energy. As activity becomes so intense that the normal aerobic system does not have enough oxygen to produce the necessary energy, the anaerobic system kicks in. However, the anaerobic system burns carbohydrates and cannot produce energy for very long periods of time. Interval training increases the amount of time your anaerobic system can be productive, and also increases your VO2 max (maximum oxygen uptake capacity).

c.f. fartlek training