Restaurant chains
Fuji
(http://www.fuji.co.th) and zen specialize in surprisingly passable japanese food at very cheap prices at least compared to japanese restaurants almost anywhere else; rice/noodle mains are less than 100 baht, and you can stuff yourself full of sushi for less than 500 baht.
MK
And coca are near-ubiquitous chains specializing in what the thais call suki, perhaps better known as "hotpot" or "steamboat". a cauldron boils in the middle of your table, you buy ingredients 10-30 baht a pop and brew your own soup. the longer you spend, the better it tastes, and the bigger the group you're with, the more fun this is!
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 and some semblance of hygiene. All the chains are heavily concentrated in Bangkok, but larger cities and popular tourist spots may have an outlet or two.
S&P
(http://www.sandp.co.th) outlets are a bakery, a cafã© and a restaurant all rolled into one, but their menu's a lot larger than you'd expect: it has all the thai mainstays you can think of and then some, and most all of it is good. portions are generally rather small, with prices mostly in the 50-100 baht range.
Yum Saap
signs in thai; look for the big yellow smiley logo is known for their thai-style salads yam, but they offer all the usual suspects as well. quite cheap with mains around 50 baht.
And yes, you can find the usual McDonalds, KFC, Pizza Hut, Komalas etc if you insist. If you do end up at McD's, 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.
Kuaytiew Ruea Siam
signs in thai; look for the boat-shaped decor and hungry red pig logo does dirt-cheap noodles with prices starting at 25b. portions aren't too generous, but at that price you can get two! no concessions to english speakers in menu or taste, so point & choose from the pictures and watch out for the spicier soups.
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 Thai fried noodles cooked at a street stall or as expensive and complicated as a $100 ten-course meal by a royal chef served in one of Bangkok's 5 star 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.
etiquette
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.
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 center 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 girl/boyfriend be beautiful"!
Food is also generally brought out a dish at a time as it is prepared. It is not expected for 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 meal as it arrives.
vegetarian food
Vegetarians won't have too many problems surviving in Thailand, with one significant exception: fish sauce à¸à¹à¸³à¸à¸¥à¸² 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:
phom kin je m / di-chan kin je f à¸à¸¡à¸à¸´à¸à¸±à¸à¸à¸´à¸à¹à¸ "I eat only vegetarian food"
karunaa mai sai naam plaa à¸à¸£à¸¸à¸à¸²à¹à¸¡à¹à¹à¸ªà¹à¸à¹à¸³à¸à¸¥à¸² "Please don't use fish sauce"