One way to start organizing our thoughts about the fruit world is to consider the flowers from which the fruits develop. Below, the flower arrangements required for three basic kinds of fruits are sketched.
In the sketch, the pink parts are the flowers' corollas, and the green parts are pistils, each pistil consisting of an egg-shaped ovary with its neck-like style and divided stigma.
Simple fruits derive from flowers with just one pistil, like our Standard Blossom. Most fruits we encounter in our backyards are simple fruits. All legumes (beans and peas) are simple fruits, as are the follicles of milkweeds, the capsules of poppies, the achenes of dandelions, the nuts of oaks, and edibles such as tomatoes, grapes, avocados, eggplants and red peppers. Even simple fruits with easy-to-recognize fleshy parts and seeds can can be deceptive, though. In the picture, a Husk Tomato fruit, Physalis ixocarpa, hangs suspended inside a bladdery husk, which is the earlier flower's much-expanded calyx. Therefore, the calyx/bladder, because it doesn't start out as part of a pistil, isn't part of the fruit.
Aggregate fruits differ from simple fruits in that they derive from a single flower bearing more than one pistil. The single flower's several matured pistils "aggregate" to form the mature fruit. Examples include blackberries and strawberries. At the right, three aggregate fruits of a Southern Dewberry, Rubus trivialis, are in different stages of maturity. Each fruit originated from a single flower, and if you look very closely you can see that each round item in each dewberry bears a tiny, dried-up style in its center. That makes sense because each spherical part of the fruit began as a single pistil bearing a style and a stigma.
Multiple fruits differ from the above in that they derive from several individual flowers grouped together in such a way that their maturing pistils fuse into one multiple fruit. Pineapples, mulberries, Osage Orange and breadfruit are all multiple fruits. At the right, a developing multiple fruit of the medicinal Noni, Morinda citrifolia, has individual white flowers with their inferior ovaries packed together into a tight cluster, almost like a green pine-cone. Once pollinated, the flowers fall away, leaving a scar atop the ovaries. Once the crammed-together ovaries mature, they form a white, succulent multiple fruit that, except for the scar atop each ovary including a ring where the corolla fell off, looks like a single aggregate fruit. An aggregate fruit may have a dried-up style and stigma atop its individual bumps, but not such a corolla scar.