A howitzer is a cannon, and looks like a regular artillery piece. However, the howitzer is used by firing indirect fire, NOT directly at the enemy. A "Gun", another cannon, an…d looks like a regular artillery piece is designed to aim directly at the enemy..
Example (1): A Navy battleship's 16" gun is designed to aim directly at another battleship, with DIRECT fire..
Example (2): An Army's 105mm howitzer is designed to lob (in an arch) it's shell into an area it's gun crew cannot physically see (INDIRECT Fire). The 105mm artillerymen have fired their howitzer by the numbers given to it by Forward Observers (FO's) either flying over the battlefield or walking with the grunts. ( Full Answer )