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 for both the Up and Down positions but just a single Down switch will work. They can be arranged as single targets or 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.