As Johnny J says - fartlek is not really structured, just a run, usually off road, where you vary things up - some v fast sprint bits, some a bit longer at a fast pace, some hill reps & some slow/jog. I believe the term can be translated "speed play".
Intervals are more structured, and can either be all the same distance at the same speed with a recovery, or could also be a "pyramid session" where the intervals get longer (or faster) and then back down to the original one.