Modern artillery ordnance ammunition is generally of two types: separate loading and semi-fixed. Semi-fixed ammunition (rounds) appear in the form of a projectile mated with a cartridge case which contains the propellant and they resemble small arms rounds.

The canister is outfitted with a primer on its base which fires upon contact from the firing pin. Gunpowder, precision machined to burn evenly, is contained inside of cloth bags that are numbered. US/NATO 105 mm howitzers use semi-fixed ammunition, containing seven powder bags referred to as increments or charges. Putting the powder in bags allows the howitzer crew to remove the increments when firing at closer targets. The unused increments are disposed of by analysing burning in a powder pit at a safe distance from the guns.

Above a certain size, semi-fixed rounds are impracticable; the weight of the whole assembly is too much to be carried effectively. In this case separate loading ammunition is used: the projectile and propelling charge are supplied and loaded separately. The projectile is rammed home in the chamber, the powder charge(s) are loaded (usually by hand), then the breech is closed and the primer is inserted into the primer holder on the back the breech. Separate loading ammunition is typically used on 155 mm and larger howitzers. Several propellant types are available for 155 mm howitzer.

All normal projectiles arrive at the weapon with a plug in the fuze well on the nose of the projectile. Using a special fuze wrench, the plug is unscrewed and a fuze is screwed in. The decision as to which type of fuze to use is made by the fire direction center and carried out by the gun crew.

The armaments fitted to early tanks were contemporary field or naval artillery pieces and used the same ammunition. When tank versus tank combat became more important, and specific tank guns did not exist, it became common to adapt anti-aircraft guns (artillery) which fired shells of high velocity, which were needed for high altitude targets. As the armour applied to tanks increased, ammunition for tank use paralleled that of anti-tank guns. Current tank gun ammunition is a single fixed round ("shell" and charge combined in a single piece) for quick loading, the propellant is in a combustible case, thus negating empty shell casings. The primary anti-armor (anti-tank) warhead is the sabot round, a shaped charge or sensor fuzed warhead.

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http://en.wikipedia.org/wiki/Shell_(projectile)