Cams
Cams come in different shapes and sizes. Their main job is to convert rotary motion (round & round) into reciprocating motion (backwards and forwards or up and down). Resting on the cam is a follower or rod. As the cam rotates the follower moves up and down. In the diagram three cams are shown. With the pear shaped cam when the narrow part is at the top any rod or follower will be pushed up and when the  narrow part is at the bottom the follower will drop down. With the heart shaped cam the follower will have 3 highs for every turn of the axle. The circular cam will produce a similar effect to the pear shaped cam.

Cranks

Crank mechanisms can be used in two ways, to turn rotary motion into reciprocating motion or turn reciprocating motion into rotary motion. For example the explosion inside the cylinder of a car pushes the piston down and the connecting rod (follower) pushes the crankshaft round, which in turn drives the wheels of the car.