The difference between howitzer and cannon is mainly in function, which can also be seen through appearance.
A cannon is a large-caliber gun classified as a type of artillery and usually launches a projectile using explosive chemical propellant. While a howitzer has a thick barrel and generally a large ranged weapon between an artillery gun which has smaller, higher-velocity shells fired at flatter trajectories.
The term “Cannon” appeared around 1670, when it was defined as “a cannon that can be fired from a carriage or a fixed support, with a barrel larger than one inch”. Later, the cannon was specifically referred to as a direct-fire artillery with a length-to-diameter ratio and used for flat ballistic attacks, and it has been used until modern times.