Mussaman curry is probably the most 'un-Thai' style of Thai curries. It's more like a stew than other Thai curries. The word mussaman has no meaning in Thai, other than as the name of this curry. It may be, like several words in the modern Thai language, a corruption of a foreign word, probably Persian if the stories around this recipe are to be believed. The legend of the dish's origin is that it is derived from a recipe bought by the first Persian ambassador to the Court of Ayutthaya (the capital of old Siam).
Green curry is perhaps the most ubiquitous of all Thai curries. You'll find it on the menu in practically every restaurant in the kingdom, and it makes a frequent appearance at the curry stalls. Although the dish has its origins in the central plains, it's found and appreciated throughout the country. Much of the attraction of green curry is its flexibility. It works well not only with rice, but is also quite popular as a topping for the spaghetti like rice noodles called khanom jeen. It has also proved quite popular for adding a Thai twist to Italian pastas.
Eggplants are very important in many Thai dishes, although you could say their role is more of a "supporting" character. Eggplants are thought to have originated in South-east Asia, and you can still find a seemingly infinite variety of eggplants in Thai markets. Eggplants apparently made their way to India by ancient trade routes, and from there to the Mediterranean.
This is one of those "standard" dishes you'll find in almost every Thai restaurant. When ordering ala-carte dishes, Thais will normally include one or two vegetable dishes, and this mixed vegetable recipe is the easiest choice; one all guests can agree on.
You can, of course, vary this dish to use whatever you have on hand. It's one of those recipes that you rarely make exactly the same way twice. To make it truly vegetarian, use mushroom sauce in place of oyster sauce.