For smoothbore guns (the 12 pounder bronze field gun/howitzer was the most common), there were four kinds of projectile, the solid shot, the shell, spherical case, and canister. The first three are round balls, the first is simply a solid iron ball. The shell and spherical case contain powder charges touched off by a fuse inserted in the projectile. The shell is a hollow ball filled with powder; damage is done mainly by the explosion. Spherical case, also known as Shrapnel*, after its British inventor, has small iron balls inside it along with a powder charge; on exploding, the balls are blown out and kill or injure personnel. Canister is what it sounds like, a can full of iron balls used to convert the gun into a large shotgun; it was mainly a short range anti-personnel weapon used against attacking infantry or cavalry.
Shell and spherical case were issued attached to a sabot with the fuse pointing forward. It was cut to the appropriate time before loading and ignited by the flame from the gun's charge washing around the projectile. (No, the fuse didn't point backward, as some folks think; the gun's charge would have blown it into the shell and exploded the shell in the bore, a very undesirable situation.)
Explosive projectiles from rifled guns usually had impact fuses, meaning that the charge in the shell or case shot was ignited by a percussion cap when the projectile struck something, though time fuses were also available for some projectiles.
From the safety viewpoint today, solid shot is no danger; it is simply an iron ball. But if a ball has a fuse in it, it is a shell or spherical case and is highly dangerous to mess with. Sometimes heavy rust, dirt or some kind of accretion may cover up the fuse and give the appearance of a solid ball. If in doubt, call in the experts as mentioned in the earlier link. All elongated projectiles should be treated as explosive unless one has expert knowledge of the types of projectiles used in the Civil War era.
*The term later came to mean shell fragments, but that was not the original meaning nor was it Shrapnel's invention, which was for balls within a shell.
Jim