A howitzer is a type of artillery piece characterized by a relatively short barrel and the use of comparatively small propellant charges to propel projectiles at relatively high trajectories, with a steep angle of descent. Such weapons have barrel length 15 to 25 times the caliber of the gun.
In the terminology used by European till 20th century, the howitzer stood between the "gun" ( longer barrel, larger propelling charges, smaller shells, higher velocities, and flatter trajectories) and the "mortar" (used to fire at higher angles of ascent and descent).
By the early 20th century the differences between guns and howitzers is explained as follows:
Guns - higher velocity and longer range, single charge propellant, maximum elevation generally less than 35 degrees.
Howitzers - lower velocity and shorter range, multi-charge propellant, maximum elevation typically more than 45 degrees
Since the First World War, equipment that has the features of gun and the earlier howitzer appeared. Thus, the category of gun-howitzer - relatively long barrels and high muzzle velocity combined with multiple propelling charges and high maximum elevation came into being. But in USA, gun-howitzers have been officially described as "howitzers" for more than sixty years. So "howitzer" is being used now as a generic term for any kind of artillery piece that is designed to attack targets using indirect fire.
Since World War II, howitzers combining the earlier guns and howitzers have become popular artillery weapons.
http://www.marines.com/operating-forces/equipment/weapons/m777-howitzer
