A sling is a device used to fling a rock or piece of lead shot at a target. Then are known from antiquity and units of slingers were apparently used at such times. It is believed that the Biblical David was a highly skilled slinger of such ilk.
Typically a sling has a leather pouch which cradles the projectile and two straps that extend from either side of the pouch.
The projectile is added to the pouch, then the sling is then swung around in a circle and one of the two straps released. The projectile then escapes and travels off with its accumulated motion.