First what it is not.   It is not what one typical envisions hearing the word clutch.   

It is probably best described by its function. A one direction drive with an over/ride.

To illustrate.

Take a ratchet wrench representing the starter, attaching an extension for the jack/shaft and a socket representing the pinion / ring gear.

Holding the socket in one hand while turning the ratchet with the other illustrates the starter cranking the engine.

To represent the engine firing, turn the socket in the same rotation and notice that the ratchet (starter) disengages.

Note; while the "ratchet" illustration is useful for demonstrating over /ride, the comparison stops there since most ratchet mechanisms consist of a pawl and gear not a Sprague roller or ball assembly...