Fartlek (excuse me), or interval training is a technique that not only saves time but also gets results. In its most simplest form, interval training consists of short bursts of intense work immediately followed with active recovery.
The varieties of interval training are endless. Depending upon your goal (cardio, fat loss, muscle strength, etc.) you can design a training routine that is unique for you.
Interval training is a great training tool to use if you are wanting to get the most out of your workouts and time.