Fartlek, which means "speed play" in Swedish, is continuous training with interval training.
Fartlek runs are a very simple form of a long distance run. Fartlek training “is simply defined as periods of fast running intermixed with periods of slower running."
For some people, this could be a mix of jogging and sprinting, but for beginners it could be walking with jogging sections added in when possible. A simple example of what a runner would do during a fartlek run is “sprint all out from one light pole to the next, jog to the corner, give a medium effort for a couple of blocks, jog between four light poles and sprint to a stop sign, and so on, for a set total time or distance."
The variable intensity and continuous nature of the exercise places stress on both the aerobic and anaerobic systems. It differs from traditional interval training in that it is unstructured; intensity and/or speed varies, as the athlete wishes.
Fartlek training is generally associated with running, but can include almost any kind of exercise.
For more, check out this article written by Trainer & Assessor, Alicia Turner: Fartlek Training