A type of anti-personnel Ammunition, Cannister Shot has a shorter range than Grapeshot but is generally more effective at close range. It consisted of a closed cylindrical metal canister typically filled with round lead or iron balls, normally packed with sawdust to add more solidity to the mass and to prevent the balls from crowding each other when the round was fired. The canister itself was usually made of tin, often dipped in a lacquer of beeswax diluted with turpentine to prevent oxidation and rusting of the metal. Iron was substituted for tin for larger caliber guns. The ends of the canister were closed with wooden or metal disks. Attached to the back of the metal canister was a cloth cartridge bag, which contained gunpowder as the impetus to expel the round from the gun tube. A sabot of wood, metal, or similar material was used to help guide the round during its expulsion from the cannon.
When it was fired, the canister would disintegrate and its shards and projectiles would spread out in a conical formation, causing a wide swath of destruction. My source for this is "Tin Case-Shot or Canister Shot in the 18th Century" by Adrian B. Caruana http://www.militaryheritage.com/caseshot.htm