A cannon is a type of artillery, or large gun. They are made with a large tube called "barrel", and shoot projectiles. Cannons were first made in China, and were used with another Chinese invention, gunpowder. The gunpowder was put at the back of the tube, and the projectile, usually a cannonball, was placed in front. The gunpowder was set on fire, and would explode. This pushed the cannonball out the front of the tube towards the enemy. Howitzers and mortars are similar to cannons but they have shorter barrels and shoot the projectiles (that is: balls or shells) higher.
A field gun is a cannon if its barrel lenght is more than 35 times its caliber (the inner diameter, "bore", of its shell pathway). If the barrel length is 10 to 35 times the caliber, it is a howitzer. Less than 10 times are called mortars.