One of the main differences between different machine gun models is the loading mechanism. The early manual machine guns, such as the Gatling gun, used a device called the ammunition hopper. Hoppers are just metal boxes containing loose individual cartridges that fit on top of the machine gun mechanism. One by one, the cartridges fall out of the hopper and into the breech. Hoppers can hold a good amount of ammunition and they're easy to reload even while the gun is firing, but they are fairly cumbersome and only work if the gun is positioned right side up.
The hopper system was replaced by the belt-fed system, which helps control the ammunition's movement into the gun. Ammunition is contained on a long belt, which the operator holds, or is contained in a bag or box. After a round is fired, it moves out of the way, and a new round slips into place.
Another system is the spring-operated magazine. In this system, a spring pushes cartridges in a magazine casing up into the breech. The main advantages of this mechanism are that it's reliable, lightweight and easy to use. The main disadvantage is that it can only hold a relatively small amount of ammunition.
Read on for more information about the belt system's advantages.