Runners who want to improve their speed are well served by incorporating interval training after they have a good base mileage. Interval training and fartlek training are very similar. There are times when these terms are used interchangeably but they are different.
Interval training is based on doing speed sprints in an organized and planned fashion. For instance there is a ladder workout where the athlete does sprints on the track at distances of 400, 800, 1200, 1600, 1200, 800 and finishing with 400. The athlete will jog and cool down between each of the sprints.
Fartlek is the Swedish word for speed play. When an athlete does a fartlek run it is often best done with two or three running partners. The group will do a strong warm up jog of a mile together and then the lead runner will do a sprint of undetermined length with the rest following. After the sprint is completed the lead runner drops to the back of the pack and the pack does a cool down jog. The new lead runner will then do another sprint of undetermined length and the cycle repeats until the runners have come to the agreed finishing point. The point behind a fartlek run is speed play. The runners are playing at speed sprints and having fun, which is what running is all about anyway!