Plungers, called pistons, move in or out when they are pushed or pulled. Hydraulic systems are used instead of electric ones when there can be danger from sparks. For example, in the painting area of an automobile factory sparks could ignite the fumes. Pneumatic systems are often used in grippers, the part of a robot that picks something up. Gas can be compressed, making the gripper more responsive and less likely to crush what it's picking up.
Air muscles are simple pneumatic devices that contract by thickening, a lot like our own muscles do.
Since they are mostly made of soft plastic and rubber, air muscles can be bent around curves and can even work underwater. Vacuum grippers use suction to pick up fragile things like sheets of paper or glass. Electromagnets use electricity to pick up and release steel or iron objects.