How is a cannonball fired from a cannon?

Since you've used the term "Cannon Ball" I figure you're asking about the old muzzle loading type.
The Cannon is pretty much just a big tube with one end blocked off. An amount of explosive is placed inside the tube, a ball is then placed in on top of the explosive.
There is a very small hole in the sealed end of the tube where you insert a fuse or touch with a match igniting the explosive.
The resulting blast propels the ball out the end of the tube.