You'll find variations in English spellings, with g and k interchanged because k is pronounced like g in good (ie, gaeng or kaeng, gai or kai), and p and ph interchanged because there is no f sound in Thai (as in pad or phad). Spoken Thai can be very tricky, so if you plan to visit the markets and travel off the beaten track, be sure to invest in a Thai phrase book or dictionary with a good pronunciation guide.
Although over-simplified, here are a few tips that helped me untangle Thai food terms:
Start with the methods of cooking: gaeng is curry, pad means fried, tom means boiled. Now, the key ingredients: gao is rice, gai is chicken, moo is pork, pla is fish, gung is prawn.
So, "gao-tom-gai" will get you boiled (steamed) rice with chicken, and "pad gai" will get you fried chicken; "gaeng gung" will get you curried prawns; "gao-pad-moo" will get you fried rice with pork. (My trick for remembering pork was to picture a pig mooo-ing like a cow!)
Of course, there are a few types of Thai curry, and alot more fresh ingredients that you'll discover, especially if you start cooking Thai. The Recipe for Travel has a small sample menu of some of the more popular dishes you'll find while travelling in Thailand, and for that matter, in many Thai restaurants throughout the world.
If you're just starting to cook Thai food and you're not familiar with some of the ingredients in the dishes you find here, invest in a good cookbook that covers all the cuisines of Southeast Asia and includes a glossary of Asian ingredients, because many of them cross cuisines but go by different names.
Once you start stocking up on ingredients for cooking Thai food, you'll be able to use many of them in Vietnamese, Malaysian and Indonesian dishes too! If you're going to travel in Thailand, be sure to explore the markets and watch all the cooking -- immerse yourself in the street food culture.