What is a drop set? when you are doing a set of 8 to 12 reps and then you pull off some of the weight about 15-20% of the weight off and you do another set immediately. It allows your muscle to get as much blood into it as posibble and achieve maximum fatigue.
How much rest between sets should you have? 2-5 minutes rest between sets. some times when he feels like he has more energy he has less rest, but it all depends on the diet and on the individual. I personally like to take 1 minute rest for exercises that I focus more on definition and about 2 minutes rest on heavier weight exercises where I focus more on gaining mass.