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.

Cake is a form of bread or bread-like food. In its modern forms, it is typically a sweet baked dessert.