Mazzard is a variety of true sweet cherry (Prunus avium), so it looks exactly like any other sweet cherry tree wrt leaves and bark. If left to grow it will fruit and produce sweet cherries (yellow-reddish cherries that are small and not very flavorful but completely edible).
Mahaleb is a different species (Prunus mahaleb) with inedible fruit (the fruit has a very bitter flavor). Its leaves are wider and shorter than those of a sweet cherry:
