Block and Tackle
A block and tackle is a rope and pulley system that uses doubled-up (or more) ropes in order to "gear down", allowing you to reduce the amount of force used to lift a weight (usually in a suspension).
To simplify, in a 1:1 pulley system, to lift a 100 pound weight one foot, you will need to apply 100 pounds of force on the end of the rope for a distance of one foot. However, when using a 2:1 pulley system, to lift a 100 pound weight one foot, you will need to apply only 50 pounds of weight, but over a distance of two feet (that is, twice the distance, but half the weight) — basically it's the same as using a lever to lift something. The ratio used above to describe the amount of weight being lifted versus the weight applied is often refered to as the "mechanical advantage" (MA) of the system.
The forces exerted in suspension can be significant. A good block and tackle system allows a single person to safely lift and control large weights.