Grapeshot was large lead, but more often iron, balls fired out of a cannon similar to firing shot from a shotgun.
Cannister shot was grapeshot, packed into a lead, typically, " can", and fired from a cannon. The cannister would split open, letting the shot spread out, but at distances further than the muzzle of the barrel. Think of a plastic shotcup in a shotgun, and you see the same idea.
It allows the grapeshot to get down range before spreading out to kill troops. Cannister shot was fired at an elevated angle, so that the can came apart higher off the ground than the height of the muzzle. Grapeshot was used when the enemy was at very close range.
Volley guns were not very popular, although they saw some use. Yes, they consisted of a number of barrels fixed to a frame, and usually to some kind of wheeled carriage. When the fuse was ignited, all the barrels fired off at once. If the volley gun as pointed correctly, the multiple shots could be very deadly. But, if the volley gun was NOT pointed correctly, then NONE of the shots were effective. AND, the more barrels, the longer it took to reload( and clean). Volley guns were even seen in use as late the civil war by some state militias. To my knowledge, they were not in the inventory of any unit in the U.S. Army during our civil war. Unlike cannons, that had crude sights or sighting systems, volley guns had no such sights. The gun was difficult to make, because aligning all the barrels to shoot to the same place based on the outside dimension of the barrel required bores be "Plumb" with these outside flats. Not until the 1850s did industry have the tooling to produce anything close to this kind of uniform barrel. By then, the first breechloading, and rifled barrel parrot cannons were being made, and they were far quicker to load and clean, and could be moved around a battle field just as easily as a volley gun, and proved much more effective, with sights, and cannister shot, than the slow volley guns.
Dr. Gatling actually took the volley gun idea and improved it with his revolving barreled, " gatling gun, which saw limited service at the end of the Civil war in 1865. The Gatling gun was constantly improved after that, using breechloading, cartridge ammo, in the Crimean War, the Wars in S. Africa, and Asia, and finally in the Spanish American War, where it was being replaced by the first Colt, and later Browning designed machine guns.
Note, all these rapid fire guns, from the volley gun to the Gatling gun, were invented by men who hoped the devastation cause by these firearms would bring an End of War, because no one would resort to war to resolve political disputes knowing how many fine men would be killed so quickly. Dr. Gatling would be both horrified, and perhaps a bit delighted, to see " puff the magic dragon", the electric motor driven high speed gatling guns used on helicopter guns ships, today by our armed forces. He would not wish the level of destruction such a weapon causes, but he would be interested in knowing his idea has actually made a gun that can fire more rapidly than mere single barrel machine guns, with accuracy.The volley guns simply didn't have any accuracy.