From Team 449 Wiki
Pneumatic cylinders, or pistons, have a rod in them that moves in and out when air is put in. Single-action cylinders have only one air input, which drives the cylinder one direction. They also have a spring on on the inside to return them to their original position. They can be either spring-extended (rod is naturally out) or spring-returned (rod is naturally in). Double-action cylinders are more commonly used, and are generally preferable to single-action cylinders. They have two air inputs, generally one at each end. When air is put into the bottom, the rod extends; into the top, the rod retracts.
A cylinder is primarily characterized by its bore diameter and stroke length. The bore diameter is the inner diameter of the cylinder. The stroke length is the difference in length between the extended and retracted positions.