Regional Thai Cuisine
There's really no one Thai cuisine. "Thai food" is best thought of as an umbrella term, encompassing the cuisines of four regions: the north, the northeast, the south and the central plains.
Thailand's location, central to many trade routes, lent itself well to assimilation of ingredients and techniques from all over the world. Stir-frying and steaming were adopted from China, curries from India, and braising from what are now Myanmar and Cambodia. Ingredients came from all over the world as well. Peanuts and chiles, both native to the New World, are relatively recent additions to the Thai kitchen that quickly made themselves integral to the cuisine.
Northern Thailand's cuisine is predominantly hot and salty, though more moderate in both than food from other regions. Because it's a temperate region, it's not quite hot enough for coconuts to thrive, so dishes generally tend to be moistened with water or broth, not coconut milk. Heavy forestation means there's lots of fuel for cooking, so grilling is popular, as are long-cooking and braising; the forest also provides a number of bitter herbs, lending the cuisine a slightly astringent profile. Pork is popular both as a meat and as a cooking medium; frying in pork fat is common.
Northeastern Thailand, in contrast, has far less fuel for cooking; there's substantially more cured and raw food than you'd see in Northern Thailand. The region, Thailand's poorest, has struggled with farming and the effects of deforestation; the cuisine has adapted to reflect this. That means that dishes tend to be extremely hot and pungent so that less of them are needed to flavor their accompanying rice. Grilling and boiling are the predominant methods of cooking; soups made of preserved fish are common as well.
Southern Thailand's proximity to the Gulf of Thailand means fish plays a huge role in the cuisine, including prawns, crabs, oysters, squid and mackerel. Whatever can't be eaten immediately is dried in the hot sun; the shrimp paste produced by this method is the cornerstone of most Southern relishes. Southern food is Thailand's spiciest; the flavor profile is primarily hot, sour and salty. Coconut oil and milk are used heavily in curries, as are fresh herbs and fish.
Central Plains' food is probably the most familiar to Westerners, as most American Thai restaurants serve primarily Central-Plains-style food. Proximity to Bangkok has meant a phenomenally diverse array of available produce (most of it cultivated); flavors tend to be highly complex and layered, with the dominating flavors being hot and salty. The primary meats are chicken, duck and pork; prawns and freshwater fish are readily available as well. Limes provide sour contrast to curries and brothy soups. Chinese influence means lots of stir-fries and noodle dishes; and the curries you see in almost every Thai restaurant in the US (red, green and massaman) have their origins in the classic fried curries of the Central Plains.