Having recently started a marathon training plan and struggled to know the difference between a tempo run and a fartlek, much less why I would want to do one or the other I found yesterday's post by Jason at Strength running to be very useful. He's created a free e-book called "52 workouts, 52 weeks".
This aims to provide the reader with some variation in a stagnated schedule by offering a variety of workouts from long runs to sprints. As Jason says, doing one per week and working through the book is not the name of the game; rather we should pick routines that suit the goal in mind and use them appropriately.
To me though this book also serves to underline what a tempo run should represent (i.e. how hard compared to a normal run), how exactly to complete a run that my plan describes as "Start slow, finish fast" and just how many seconds I should consider sprinting up a hill before I can stop and jog back down again.
So its really a good tool for beginners, uninformed intermediates and jaded experts it seems.
The e-book is here