Two exercises done in succession, usually pairing agonst muscles such as the biceps/triceps. While the biceps flex the forearm, the triceps extend it. These two different functions are therefore opposite eachother and do not require the same muscles to perform. Thus, one muscle group does not become fatigued.
Supersets are often used to conserve time or up intensity.
Does the amount of reps matter (like a 12 then a 12, 12 to a 5?
Or is any lift followed by any lift w/ no rest a superset?