A Drop Target is a flat mechanical target that is held up on a small ledge and when hit with a ball, is moved backwards and dropped down via a small spring. There are generally switches in both the Up and Down positions. They can be single targets or arranged in rows.

Every manufacturer has their own designs and use a variety of switch types to detect the target position - Williams used optos while modern Stern games use microswitches.

Diagram of a standard Williams target including a drop coil:

They are also available in an Inline format.