Supersets are two different exercises done without resting. They may be antagonist, such as tricep pushdowns and standing bb curls. The idea is no rest between sets, and usually only about 30 sec. or so between supersets. Another example may be Bench presses with lat pull downs to work chest and back. Should get a pretty good pump out of them.
yea, its very good endurance too. i tried them in a couple of sessions , and without doing any cardio, my shirt was soaking wet. its like cardio and weights in one :)