Most people associate the word "Fruit" with succulent fleshy plant structures but botanically the concept is much wider than this. All fruits develop from a flower - the classification of which is based on which parts of the fertilised ovary expand in size. The fruit structures have evolved in various ways to enhance wide dispersion from the initial plant and improve the chance of seed germination. The basic classification of fruit types is:
A. Simple Fruits formed from a single ovary which may contain more than one seed.
(i) Fleshy fruit types - berries, drupes and pomes
(ii) Dry fruit types - legumes, follicles, nuts & capsules
B. Aggregate Fruits formed from a number of ovaries in one flower fusing into a single fruit
e.g. strawberries, raspberries and blackberries.
C. Multiple Fruits formed from several flowers having the ovaries fused into a single fruit
e.g. mulberries, pineapples and figs. Multiple Fruits can be difficult to visualise but if the fruit development is observed on the plant, justification for the classification can be seen.
BERRIES have a fleshy pulp containing the seeds centrally with a thin skin often coloured conspicuously to attract birds and animals. Typical examples are the grape and tomato but also included is the capsicum, carambola, mangosteen, paw-paw, passionfruit, roseapple, sapodilla, avocado (oily flesh) and banana (seeds very rare). The citrus family are special berries called Hesperidia the name coming from Hesper a Greek mythological character who had a citrus grove. The skin (rind) is inedible and the inside pulp has a high juice content but all family members are similar - typical of lemons and oranges etc. The Cucurbit family are also special berries called Pepos. I think Pepois Spanish for "Gourd'' but all the family members have a hard leathery skin including cucumbers, chokoes, pumpkins and watermelons.
DRUPES are distinguished by containing a woody pit or stone which encloses one or occasionally two seeds. Drupe is a Greek word for "Olive" but possibly the Apricot is a better example with its large amount of edible flesh. Other fleshy drupes are cherries, peaches, plums mangoes and lychees. Some drupes have almost no flesh and the inner seed is eaten - examples being the Almond and Pecan. The coconut has an external covering that is thick and fibrous which promotes floating in seawater but it is nevertheless a drupe.
POMES of which Apples, Pears and Loquats are examples are so called from the French word for Apple. The base of these fruits contains the residual structure of the flower called the Calyx and the main fleshy structure we eat is called the Hypanthium which surrounds the "core" containing the seeds.
LEGUMES of which beans and peas are typical develop a pod which splits down both sides when the seeds are ripe.
FOLLICLES of which the Macadamia is typical, split down one side of the husk to release the so-called "nut" botanically described as a Locule. We crack this Locule to get at the seed typically called the Kernel.
CAPSULES to my knowledge do not produce edible structures but a good example is the Poppy head which resembles a salt shaker when ripe.
NUTS are not common - the Acorn and Hazelnuts are good examples with their special "cap" and hard leathery skin enclosing one seed. Most commercial so-called nuts (Almond, Pecan) are Drupes.