As like many famous cuisines around the world have signature combinations and flavors, there are relatively small number of ingredients that can serve to define Thai cooking with couple of overriding principles.
Standard Thai ingredients include Chilies, dried black peppercorns, Nam plan (fish sauce made from fermented fish and salt) or soya sauce, palm oil or any other vegetable cooking oil, tamarind paste (or fresh lime), Lime leaves or lime zest can be used as a substitute, Coconut milk, garlic , Galangal (a close relative of ginger), abundant fresh herbs like cilantro, mint, Thai basil and coriander and sticky rice.
Many Thai dishes are like what we call curries. Although they may contain curry powder, they are generally based on the combination of herbs and aromic vegetables rather than dried spices. A typical Thai dish will have a mixture of chilies, lime leaf, sugar, garlic, galangal (or ginger) and shallots giving it a sweet, sour and salty flavor. Herbs are used to freshen dishes, but the well balanced trinity of sweet, salty and sour is all-important. Even desserts may combine all these flavors like Sweet bean pancakes topped with chilies and a garnish of shallots and lime.
The basic principle of Thai cooking involves stir-fry or stew of your choice and flavor it with garlic, galangal or ginger, chilies, Lemon grass and lime and finish it with nam pla or soya sauce. It will be just as good as a the stuff you may be ordering from the Thai restaurant.
Rice is a staple thai food. The most popular is a sticky rice which is delicious lightly sweet that is soaked in water for about an hour and then wrapped in cheesecloth and steamed over boiling water in a covered until tender. Rice Noodles although not considered really a Thai food are also quite popular.