In an interval workout, you run a set distance or time at a fast pace, then recover with easy jogging for another set distance or time, and repeat the whole thing a specific number of times. For example, after warming up thoroughly, you could run two laps hard and then recover by running two more laps easy, and then repeat the whole thing two more times. By planning the workout ahead of time, you'll know when you've done enough. Shorten the recovery time for longer intervals, but make sure the recovery is always at least half as long as the interval. Try to keep your intervals consistent, within about 10 seconds of each other.
Fartlek is Swedish for speed play .
It refers to random burst of speed during a regular road or trail run. Once you warm up, you could do a fartlek by running hard to a landmark like a telephone poll, then running easy to another point, like a stop sign. You could also do fartleks by time instead of distance. In fact, some runners prefer to do them this way-two minutes on, two minutes off and so on. A watch with a repeating countdown function can be useful for this.