What is the difference between a sundae and a parfait?
While both are cold desserts, a parfait (from the French word for "perfect") lends itself to a bit more culinary craftsmanship. Two scoops of ice cream in a bowl with syrup and a cherry on top could be called a sundae. Such a creation would never be mistaken for a parfait.
Sundaes always have ice cream as their base. Parfaits may or may not... it could be soft-serve, frozen whipped cream, sponge cake, etc.
Parfaits are also typically smaller and served in a cup (or something similar).