Howitzer is a arty piece with shorter barrel and higher trajectory: I'd have to check on that, or someone can reply.

Found the old 18th century definition of a howitzer: "HOWITZER: A short-barreled weapon with a large powder chamber. Howitzers were lighter and fired shells with lower powder charges at higher elevations, but lower ranges than guns of the same caliber."

If you really want to get precise with it 8)(not many do) Hovitzers barrel is about/ more than 15times the bore. IE 100mm(4 inc) howitzer has about 1500mm barrel(about yard and a half for those who still live in middle ages.. 8) ) Gun has barrel lenght more than 25times the bore. Thus same 100mm piece would mean at least 2500mm barrel. Usually gun or cannon is more than 30 times the bore. Then gun-howitzer is at about 20 times the bore. Anything under 15xbore is mortar. Hope it helps..

ISOT is right about the dimensions: <15x bore: mortar >15x bore: howitzer >25x bore: cannon The reason for these differences is the employment of these weapons. Mortar/howitzer fire >45' angle to drop shells balistically on a target. The classic indirect fire weapons. Very important if you want to drop shells into the tops of enemy positions or to penetrate top cover or, given places like Afghanistan, reach just over the crest of a hill to engage reverse slope positions. Cannons fire <45' angle and often have greater velocity and range; may be smaller shells but arrive with little warning. The traditional "grazing" fire. The relationship between barrel length and muzzle velocity is not a constant; with a certain amount of charge, there is a point beyond which the shell will no longer accelerate, no matter how long the barrel. At that point a wide bore can deliver a heavy charge with adequate accuracy. Most modern 155mm guns have a dual gun/howitzer capability. By adjusting their charge with the gun in a low elevation they can deliver short, medium and long range grazing fire; then raise the elevation to fire a high angle howitzer engagement at closer ranges. Hit the forward slopes, then hit the reverse slopes.

to elaborate a bit on Worchester's comments: The charges are REALLY important. You don't want a grenade to arrive on flat trajectory(if you want shrapnell to be effective that is, if you want to put a hole through metal that's a different thing) if grenade explodes on flat trajectory, most of the shrapnell goes into air=is ineffective. If grenade explodes close to 90 deg trajectory almost all shrapnell goes here it is intended= to ground= on top anybody unfortunate enought to warrant gods wrath to rained on top of them. Usually one wishes that trajectoy is about 30deg, or more.(0 being the flat..)

Gun= high velocity, flat trajectory, long range, very inaccurate at distance. examples being naval guns 5'/54,the battleship 16'/50 and the M-107, 175mm gun. Howitzer= medium velocity, medium trajectory, capable of both low and high angle fire, medium range, very accurate. Examples , the M101/M102 105mm, any of the current 155mm and the M-110 8" howitzer. Mortar= Low velocity, high angle, short range. inaccurate