Fartlek or Intervals whats the difference?

Hope this helps, copied this from a running website. The basic diff, as you can see, is that Fartlek training is done according to how you feel, rather than predetermined by a programmed shedule:

Fartlek is similar to interval training in that short fast runs alternate with slow running or jogging recovery intervals. However, in Fartlek the running is done on the road or on parkland or bush tracks. There is no predetermined schedule to follow, but instead the athlete will set her/his own interval lengths and pace in response to their own feeling of the workload. An advantage of Fartlek is that the athlete can concentrate on feeling the pace and their physical response to it, thereby developing self awareness and pace judgement skills. Also the athlete is free to experiment with pace and endurance, and to experience changes of pace.