A "sweet treat" is a serving of sweet food: a donut, a piece of cake, a candy bar, a slice of pie, a handful of raisins, and so on. "Sweet food" is food that has sugar in it, so it's more general, for example cake, pie, candy, etc. So, sweet treats are always sweet food, but sweet food isn't always sweet treats.

The term dessert can apply to many sweets, such as biscuits, cakes, cookies, custards, gelatins, ice creams, pastries, pies, puddings, macaroons, sweet soups, tarts, and fruit salad (fruit is commonly found in dessert courses because of its naturally occurring sweetness).