A compound fruit is one that develops from several ovaries in either a single flower or multiple flowers. Conversely, a simple fruit actually develops from one ovary.
Compound fruits could be aggregate fruits; in which a single flower is responsible for the fruit so called aggregate fruit. A single flower that contains many ovaries where each ovary develops into a small fruit. These small fruits collectively forms a bunch of fruits to form a larger fruit which is called an aggregate fruit.
Compound fruits could also be multiple fruits, in which of several flowers, each with an ovary, develop into small fruits which are clustered or fused together into a larger fruit. A good example of this is a pineapple. Each section of a pineapple is an individual fruit from the individual flower, but they have fused to form the pineapple. Grapes grow in clusters, but they are not compound fruits. Each grape grew from one ovary in one flower, and each grape remains an independent fruit.