What is a superset?

In one line

A superset is a way of training where you do one set each of two different exercises in a row without resting, and rest only after both.

What is a superset?
Two exercises done back to back without rest
At the gym you may see someone finish a set of bench press and move straight to another machine without resting. This is called a 'superset'. This post explains what a superset is, how it differs from normal sets, and why people do it.
In short
One set of each of two exercises, back to back
A superset means doing one set each of two different exercises in a row, with no rest or almost no rest in between. You rest only after finishing both. Then you start again from the first exercise in the same order.
First, reps and sets
Lifting once is 1 rep, a group of reps is 1 set
Lifting a weight up and lowering it once is a 'repetition' (rep), and a group of reps done in a row without resting is a 'set'. Normally you finish all the sets of one exercise before moving to the next, resting between every set. In this post we call this 'normal sets'.
How it works
Example: bench press, then row right away, then rest
For example, you do one set of bench press, which uses the chest muscles, then right away one set of rows, which use the back muscles. That is one superset. Then you rest and repeat, starting again with the bench press.
What gets paired
Opposite muscles, the same muscle, or upper and lower body
There are three main ways to pair exercises: muscles that move in opposite directions, like chest and back; the same muscle, like bench press and dumbbell press; or lower and upper body, like squat and bench press.
Why do it?
Less rest, so the workout ends sooner
With no rest between the two exercises, total rest time goes down. So the time needed for the same workout dropped to almost half of normal sets. In studies where people kept training this way, muscle and strength gains showed no difference from normal sets.
But it is harder
Shorter rest means more tiredness
During supersets the body used more energy, and people rated the workout as harder than normal sets. That is why researchers concluded that more recovery time is needed after superset training before the next workout.
A common misconception
Does going without rest mean less training?
Overall, no. Supersets and normal sets showed no difference in total reps or in the total weight lifted (training volume). Pairing opposite muscles, like chest and back, even led to more reps, while pairing the same muscle lowered the total weight lifted.
Summary
Two exercises back to back, then rest
A superset means doing one set each of two different exercises in a row without rest, and resting only after both. It saves time while keeping the amount of training, but pairing the same muscle lowered that amount. Related terms: drop set, training volume, RPE and RIR.