The food alone is really reason enough for a trip to Thailand. Curries, fruit shakes, stir fries, fresh fish made a zillion ways - and that's just the beginning. Food in Thailand can be as cheap and easy as 25 baht pad Thai (ผัดไทย, Thai fried noodles) cooked at a street stall or as expensive and complicated as a USD100 ten-course meal by a royal chef served in one of Bangkok's luxury hotels.
Since most backpackers will be sticking closer to the first than the second, one of the great things about Thailand is that food from stalls and tiny sidewalk restaurants is usually quite safe. Unlike some Asian countries, travellers should worry more about overeating or too much curry spice than about unclean kitchens and bad food. In fact, street restaurants, where you can see what you'll get and everything is cooked on the spot can be a safe option.
Thai food is most commonly eaten with fork and spoon. Hold the spoon in your right hand and use it to eat, and reserve the fork for piling food onto your spoon. Chopsticks are only employed for noodle soups and East Asian-style dishes. Eat sticky rice with your right hand.
Thai food is meant for sharing. Everybody gets their own plate of rice and tiny soup bowl, but all the other dishes are laid out in the centre of the table and you're free to eat what you wish. Though some people believe that taking the last piece from a shared plate is considered slightly unlucky, and you may hear people make wishes for others to compensate for their own misfortune. A popular wish is that "may my girlfriend/boyfriend be good-looking!"
Food is also generally brought out a dish at a time as it is prepared. It is not expected of diners to wait until all meals are brought out before they start eating as is polite in Western culture. Instead they should tuck into the nearest dish as it arrives.
Thai cuisine is characterized by balance and strong flavours, especially lime juice, lemon grass and fresh coriander, the combination of which gives Thai food its distinctive taste. In addition, Thai food has a deserved reputation for being spicy, with hot little torpedo-shaped chillies called phrik khii nuu (พริกขี้หนู, lit. "mouse shit chillies") making their way into many a dish. Thais are well aware that these can be more than Westerners can handle and will often ask if you like it hot (เผ็ด phet). Answer "yes" at your own risk! Another condiment that features prominently in Thai cuisine is fish sauce (น้ำปลา naam plaa), a pungent and very salty sauce that is used to flavour a wide variety of dishes.
Thai cuisine can be divided into at least four distinct regional styles: Southern Thai cuisine, Central Thai cuisine, Northern Thai cuisine and Isaan cuisine from the northeast of Thailand. Chinese influences also pervade much of Thai cuisine, with many of the most famous street food stalls in Bangkok and other cities throughout the country being owned and run by ethnic Chinese.
For information about particular dishes, please see the Thai cuisine article.
Vegetarians won't have too many problems surviving in Thailand, with one significant exception: fish sauce (น้ำปลา naam plaa) is to Thai cuisine what soy sauce is to Chinese food, and keeping it out of soups, curries and stir-fries will be a challenge.
That said, Thailand is a Buddhist country and vegetarianism is a fairly well-understood concept, especially among Chinese Thais (many of whom eat only vegetarian food during several festivals). Tofu is a traditional Thai ingredient and they aren't afraid to mix it up in some non-traditional dishes such as omelettes (with or without eggs), submarine sandwiches, and burritos. Since Thai dishes are usually made to order, it's easy to ask for anything on the menu to be made without meat or fish. Bangkok features several fantastic veggie and vegan restaurants, but outside of big cities make sure to check that your idea of "veggie" matches the chef's.
Some key phrases for vegetarians:
Thailand has a large number of indigenous restaurant chains offering much the same fare as your average street stall, but with the added advantages of air conditioning, printed menus (often in English), clean storefront. All the chains are heavily concentrated in Bangkok, but larger cities and popular tourist spots may have an outlet or two.
And yes, you can find the usual McDonalds, KFC, Pizza Hut, Komalas etc. if you insist. If you do end up at McDs, at least try the un-Maclike fried chicken with McSomTam (green papaya salad). For those craving American-style pizza, try the ubiquitous The Pizza Company, which is a less expensive and (arguably) tastier local chain.