A disk cam can be thought of as a lever of variable length that changes rotary motion into an up-or-down or side-to-side (reciprocating) motion. Disk cams are often used to operate the valves in a car engine, as shown.
A cam is a mechanical device which, on rotation, imparts to another member (the follower) a regular, repetitive motion. There are two main types of cams.
Disk cams (or plate cams) are curved, often ovoid, plates mounted on a shaft (the camshaft). On rotation the cam pushes the follower in a direction perpendicular to the shaft. (The follower is returned to its position by gravity or a spring.
Cylindrical cams consist of parallel raised lips on the surface of the camshaft and angled such that, when a projection of the follower lies in the groove so formed, rotation of the camshaft imparts to it a motion (usually to and fro) parallel to the shaft. Many engines make use of one or more camshafts: notably, in some forms of internal-combustion engine camshafts are used to regulate and actuate the cylinder valves.