What is a drop set?

In one line

A drop set is a way of doing a set where you lift a weight until you cannot do another rep, then lower the weight and keep going without rest until you cannot do another rep again.

What is a drop set?
A set that keeps going as the weight goes down
At the gym you may see someone switch from a heavy dumbbell to lighter ones, one after another, without resting. This is called a 'drop set'. This post explains what a drop set is, how it differs from a normal set, and why people do it.
In short
Until you can't, lower the weight, go again
A drop set means lifting a weight until you cannot do another rep, then lowering the weight without resting and going again until you cannot do another rep. The name comes from 'dropping' the weight.
First, sets and failure
A group without rest, and the point you cannot lift
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'. The moment in a set when you cannot lift even one more rep is called 'failure'. A drop set starts from this point of failure.
How it works
Example: failure at 20 kg, then straight to 16 kg
For example, you lift a 20 kg dumbbell until you cannot do another rep, then switch to 16 kg without resting and go to the end again. In studies, each drop usually cut the weight by about 20%, and people dropped the weight from once up to four times.
How it differs from a normal set
No rest between sets
Normally you rest for a few minutes after a set and do the next set with the same weight. In a drop set you do not rest, apart from the few seconds it takes to change the weight. So it looks like one long set where only the weight goes down.
Why do it?
Less time, similar muscle growth
With no rest periods, a drop set workout finishes quickly. In studies that timed it, 1 drop set took between 1/2 and 1/3 of the time of 3 normal sets. Muscle growth showed no clear difference from normal sets.
A common misconception
Is it easier because the weight goes down?
No. People who did drop sets rated the workout as much harder than normal sets. The weight goes down, but you keep going without rest with muscles that are already worn out from reaching failure. That is why it is seen as a method that needs more attention to recovery afterward.
Summary
At failure, lower the weight and keep going without rest
A drop set means going to failure, then lowering the weight and going to failure again without rest. Even though the weight goes down, it feels harder than a normal set. Related terms: RPE and RIR, training volume, progressive overload.