Just learn from some joshm367 who posted in HERE back in 2004 a few of the differences between Fartleks and intervals.
- Fartleks are based on time and can be done on roads or track.
- Intervals are based on distance and usually done on the track or marked course.
- Fartleks are continuous running and intervals usually have breaks.
- Intervals on the track gives you specific times to compare to the past. It also usually lets you run faster given the track surface and breaks in between intervals.