Fartlek took me ages to work out as well. RW used to assume that all readers knew what it was (but they have explained it a couple of times in the last few months!)
Anyway, it's running at different speeds - i.e. your 40min fartlek will involve running slow for a few minutes, then a fast burst for a minute or two, then a slow bit, then a medium bit, then fast again - no strict pattern, just play around. That's how I do it anyway!
APparently it's Swedish for 'speed play' or something....anyone confirm that??