A belt is a series of firearm cartridges attached together in some way by an external device to allow them to be fed into the action of a weapon more easily. Belts are typically articulated between each round, meaning they can be folded easily: this makes them simpler to carry and less prone to damage than solid feed strips.
The first devices which could be thought of as belts were articulated sets of multiple chambers and were used by chain revolvers and chain rifles, though this system of operation was rare. The first successful employments of the concept were typically strong cloth with small loops resembling the loops at the top of a set of curtains. Rounds were inserted into the loops and the belt fed through the gun. This design persisted on some weapons used in WW2, despite having a number of shortcomings related to the behavior of the cloth when exposed to both oil and other material from the weapon, and the elements: when wet, the belts would swell with moisture to the point they could no longer pass though the gun's action. Long cloth belts also had a tendency to heap up after passing through the weapon, which could cause functional issues when the weapon was fired close to the ground: period machine gun teams would often use a knife to slash the belt close to the gun so the ejected portion could be shoved aside.
This issue led to the development of metallic feed strips in an attempt to solve the problem, though these had many issues of their own.
The first metal belts for cartridges were complicated articulated feed strips with fairly severe reliability issues, but manufacturers quickly settled on simple designs which could be manufactured cheaply and in huge quantities from sheet metal. Metallic belts broadly divide into two types, depending on how the links interact with the weapon: disintegrating links are split apart when the weapon removes the cartridge from them to chamber it as they use the cartridge itself as a pin to hold the links together, while non-disintegrating links remain whole and are sometimes used to carry spent casings out of the gun. A further distinction is made between open and closed pocket links: a closed pocket link retains the front end of the round and prevents it from being pushed forwards, while an open-pocket link allows such motion.
A linkless feed is sometimes also referred to as a linkless belt: this is a powered system which has more in common with a conveyor belt, and is often used on aircraft cannons due to the dangers of ejecting belt links in proximity to engine intakes.
Hybrid belt feed is also sometimes employed. This refers to a system where a belt is used to feed some other mechanism, and that mechanism then used to bring the cartridges to the breech: for example, a revolver cannon may feed its revolver action from a belt.
Belt-feed is almost always associated with machine guns as it enables them to fire consistently for long periods without reloading, and it is also commonly used on automatic grenade launchers and less commonly on autocannons. The cost in terms of size and weight of adding a belt-feeding mechanism means it is much less common on smaller weapons, and it is impractical on very large weapons as their fire rate is typically not limited by loading speed and the weight of belt links is non-trivial.
Unlike a magazine or cylinder, a belt in itself offers almost no protection to the ammunition it carries, merely joining it together. For this reason it is common for belts to be stored in containers (referred to as belt boxes or drums (not drum magazines) for smaller devices) for protection, made of anything from cardboard to steel. For efficiency, it is common for larger machine guns to have a bracket which a belt box can be hooked onto or inserted into, allowing the gun to feed from it directly.
Belts can be loaded by hand, inserting cartridges into them one at a time, but generally this is only done when the proper equipment is not present. The typical belt-loader is a crank-operated, hopper-fed system: somewhat ironically, there is very little difference between the mechanisms of these devices and those of the crank-operated repeating guns that belt-fed self-loading weapons replaced.