Fartlek running is a non-specific interval program that allows for individually-chosen sections of higher workload and/or variable duration. While it will provide some of the benefits common to interval training, the wide variety of workouts possible places fartlek work into a different category than the strictly-programmed work/duration scheme of Tabatas.
Interestingly, the Crossfit community uses the term "Tabata" for a workout that has the same time parameters, but carries different intensity planning. They attempt to score a maximum number of repetitions for the total of all 8 20-second intervals, meaning that they attempt to keep a maximal, repeatable number of reps on all sets. The "real" Tabata protocol involves a severe lactic acid staircase, where performing at the same intensity in the last few sets is impossible. If one can perform as well in the last few sets, then the earlier ones were not performed at the proper intensity.
Lee