What is the function of a Cam?

A cam is a rotating component used in a mechanism in order to transform a movement from a circular to a reciprocating motion. A crank performs the opposite action.

A common example of a cam mechanism is that found on a car engine where the camshaft translates the rotary motion of the engine into a reciprocating motion to operate the intake and exhaust valves on the engine cylinders.

Please see the related links for an example animation and more detailed explanation.