Hmm, Webster 1913 says eggplant is "allied to the tomato". How can vegetables form alliances? Webster means related: eggplants are related to tomatoes, bell peppers, and Irish potatoes. I read somewhere that eggplants are technically considered as a berry, which seems counter-intuitive, but hey, if tomatoes are fruits, why not eggplants?
Thai has the relationship between eggplants and tomatoes built in to the language: eggplants in general are ma-kheua, while tomatoes are ma-kheua thet, thet being one way of saying "foreign"; so, tomatoes, to the Thai are foreign eggplants. Something similar happens with asparagus, by the way, which is known as no-mai farang; no-mai is bamboo, and farang is a westerner, so asparagus is westerner's bamboo.
Eggplants are native to Asia; traders brought seed westward to Europe about 1,500 years ago. Before it gained in culinary popularity, eggplant was termed "mala insana," which translates as "mad apple," owing to the belief that eating eggplant would lead to madness. Thomas Jefferson, who experimented with many varieties of plants in his Virginia garden, is credited with the introduction of eggplant to North America.
The eggplants most commonly used in Italian, French, and Mediterranean cuisines are large, inky-purple and glossy; they are tear drop or globe-shaped. These eggplants are said to contain bitter juices; thus the slices are sprinkled with salt to draw out excess moisture and bitter liquids from the flesh. Certainly the slices shed quite a bit of moisture when salted, but I've never had the courage to actually taste it to see whether it really is bitter. Joy of Cooking says only older eggplants are bitter these days, and as long as you buy an eggplant whose flesh bounces back when lightly pressed, you don't have to salt your eggplants.
French, Italian, and Greek preparation methods often rely on frying eggplant slices, which can result in an oil-sodden mess, as eggplant absorbs oil like a sponge. Instead, try lightly brushing the slices with oil and broiling or grilling till golden brown; it's much less oily that way. Another excellent preparation method is to bake the eggplant whole until the skin is dark and the flesh is soft; then, discard the peel and mash up the flesh. This is the basis for the luscious dip baba ganoush.
The many varieties of Asian eggplants do not require salting, and are used in many different ways. I particularly like the long purple plump zucchini-shaped eggplants which some people call Japanese eggplants; try slicing them in half lengthwise, brushing the cut surface with a mixture of light miso and mirin (or sake, according to sensei) and grilling till the topping bubbles. (It's called nasubi dengaku in Japanese.) Or cut them up and throw them in green curry.
Thailand has several other types of eggplant as well, most of which have a matte as opposed to a glossy skin. I've written elsewhere about the strange and bitter pea eggplant which graces bowls of green curry. I am also very fond of a small round green and white striped eggplant about the size of a golfball and containing many seeds; this one too is added to green curry, or eaten raw with tasty dips. Thai eggplants also include a long green variety which is used in dips or cut into strips and fried.