The Bomb Cannon, as its name suggests, is a small hand-held cannon which fires bombs (these need to be collected in order to use the cannon).
The bomb is fired by the player from the cannon, and is one of two in-game objects that can destroy bombable blocks. The bomb as an in-game object is fired in an arc from the player's cannon, and will pulse for a few seconds before exploding, and dealing damage to surrounding enemies and bombable blocks in range. Upon a direct hit a bomb will exlode.
In design terms, a bomb can be placed on any square not occupied by another prop, and can be set to re-spawn on any whole second between 1 and 120 seconds, or not at all.