The main distinction between these two popular rigs is how far up does the headstay go?
To the top: Masthead
Part of the way up: Fractional
Fractional rigs offer a smaller headsail luff but a much more adjustable mast. Since the headstay doesn’t meet the backstay at the head of the mast, the section between the two stays acts as a lever which can effectively bend the mast to tune the sails. All this equates to much higher performance from the boat.
Typically, fractional rigs are seen on racing boats while mast head rigs are seen on slower cruising boats.