The mechanism which holds your paintballs prior to firing is known as the hopper. Hoppers can be critical in whether your team or you as an individual are successful in your paintball missions. There are several types of hoppers, each with different mechanisms used to drop the ball and each with different capacities.
Gravity feed hoppers operate exactly as the name suggests, and are the original paintball hopper. The paintballs are stored in a compartment that consists only of a plastic holding container and a lid. The hopper is connected to the body of the gun by a feed tube, which is only wide enough to allow one ball at a time to drop in front of the bolt. The sides of the hopper are all sloped, so that balls naturally slide down to the area where the feeder tube is. Although the simple design and the cheap materials used for gravity feed hoppers make them the most inexpensive form of hopper out there, they do have several disadvantages. The first is that balls tend to get stuck in groups that jam up at the top of the tube. Although this problem can be taken care of by shaking the gun about, thus breaking up the group and allowing the ball to fall down the tube, it creates problems of its own. First, the shooter loses some valuable seconds trying to get a ball down. As all players know, this time is all that is needed for an opposing player to take you down or for you to miss an important shot. Shaking the balls around may also break them if the ball casement is weak, resulting in a jamming situation that might affect your whole gun, not to mention your usefulness in the rest of the match.
Stick Feed hoppers are located parallel to the barrel and the balls are held in a line. In order to load the balls, players need to rock the ball forward and then bring the sights to bear for the shot. A more useful model of stick feed is the vertical one, but these models can be outlawed at certain tournaments.
Force-feed hoppers are the most popular loaders among paintball hobbyists. Most of these pros have guns whose potential would be wasted if they were to use gravity as the primary loading mechanism (not to mention they themselves would be wasted by their opponents!). Force feeders have feed systems that will force balls into the gun, and grab the ball and load it instead of just mixing the balls up. Electronic models are especially useful as they can keep track of the number of rounds left in the hopper.
Agitating hoppers utilize a propeller to keep the balls from jamming up at the top of the feed tube. These hoppers still operate using gravity to bring the ball down, but the risk of having a jam is lessened. These were the first type of hoppers to be equipped with high-tech devices which would keep the shooter from firing without a ball in the chamber. This cuts down on battery usage as well as gas in older models.