A paintball marker, or paintball gun, is a device that shoots marble-sized, paint-filled balls. A hopper sits on top of the gun and the ball drops into the chamber, which then releases a burst of air through a bolt, expelling the paintball. This air can come from two different sources, either CO2 or compressed air.

Because a marker has a hopper on top and an air tank on bottom, the whole set-up can be rather bulky. Maneuvering around the course with a paint gun is usually done with the entire set-up tucked to the body as close as possible, so the weight and compactness is a vitally important aspect to a paintball marker.

The differences between a low end and high end gun are subtle, but become more important the more the advanced the game is. Power is generally not a consideration when choosing a paint gun because all commercial courses regulate the shot speed to somewhere around 300 feet per second (fps).

A high end marker is usually electric, but almost always has a higher balls per second (bps) rate. This usually ends up being around anywhere from 8 bps to as much as 25 bps. An electric trigger uses a solenoid to release the air in to the bolt, and so it becomes much easier to increase the fire rate. This usually allows certain settings similar to real war weapons such as a 3-shot burst mode, and a full-auto mode, although these settings are illegal in professional play and usually restricted in recreational play. A higher end marker almost always has a specialized barrel, which can be judged by its trueness. Higher end markers tend to operate at a lower and more consistent pounds per square inch(PSI). This is usually because a gun's accuracy depends on its ability to shoot each paintball at a consistent speed resulting in tighter groupings.

Markers not only vary in their practical aspects, but like cars, are highly customizable to their owner. Milling designs on to the metal body and/or anodizing it with a variety of colors are the most common ways to decorate the look of a paintball marker. The bolts, the grips, air chambers, and a variety of other parts can be switched out to increase performance and fit the user's preferences.

There are a few variations on the standard paintball marker, most notably the single-action pump version and the pistol version.