class of artillery which fires at a low or flat trajectory

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 balls higher.

A cannon that can be moved around on wheels. The back of the cannon is on the right

Modern guns that can shoot many big projectiles quickly and automatically are called autocannons.