What’s the difference between a crossbow and a compound crossbow?

The principal difference is that the limbs of a compound crossbow are usually much stiffer than those of an ordinary crossbow. This limb stiffness makes the compound crossbow more energy efficient than other bows, but the limbs are much too stiff to be drawn comfortably with a string attached directly to them as in an ordinary bow.

Therefore, the compound crossbow has the string attached to the pulleys, one or both of which has one or more cables attached to the opposite limb. When the string is drawn back, the string causes the pulleys to turn. This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy.

Compound crossbows generally use only pulleys that are both round and concentrically mounted, in order to capture the maximum available energy from the relatively short draw length.