A drop set is like 1 giant set where you keep dropping the weight....
For example: Leg Extensions: You can do 10 @100lbs, drop the weight immediately to 90lbs, do 10, drop the weight to 80lbs do 10 more.....all w/o stopping -- drop set....
A superset is like doing 2 exercises right after the other without stopping. Like Leg raises with crunches -- do 35 crunches, then 25 leg raises, rest 1 minute, go again....