The world of fruits is so huge and diverse that to really enjoy knowing them we need to be a bit systematic. We'll do that by thinking in terms of how the fruits develop. Here is a good insight with which to begin: Fruits are usually classified into three large groups. We can talk about simple fruits, aggregate and multiple fruits. Look:
The above diagram shows how flowers and their pistils are arranged in order to produce each of the three kinds of fruits.
Simple fruits derive from flowers having just one pistil, and therefore structured like our Standard Blossom. Most fruits we encounter in our backyard are simple fruits. All legumes (beans and peas) are simple fruits. So are the follicles of milkweeds, the capsules of poppies, the achenes of dandelions, the nuts of oaks, and such items as tomatoes, grapes, avocados, eggplants and red peppers.
Aggregate fruits differ from simple fruits in that each single flower instead of having just one pistil bears several and the matured pistils stick, or "aggregate" together to form the final fruit. Examples include blackberries and strawberries.
Multiple fruits result when several separate blossoms are grouped together so that their maturing pistils grow together to form one fruit. Here we find some really weird fruits, such as pineapples, mulberries, Osage orange and breadfruit.