Som tam is a green papaya salad which originated in northeastern Thailand but has now swept the nation, becoming popular throughout the kingdom. It's made from green papaya and a bunch of other stuff, which I'll tell you about in a moment, after I fill in all the background information I think relevant.
Som tam is quintessential street food, whipped up by entrepreneurs who emerge each morning pushing carts which they wheel over to their special destination and set up shop. Usually when you order som tam, you plonk yourself down on a rickety stool in front of a wonky folding table (previously piled on said cart), and ask for your favourite type of som tam, specifying the number of hot chilis you want added. The som tam is prepared by grinding and bruising the ingredients together in a baked clay mortar and wooden pestle - a rather bigger one than you might be used to - and then served with sticky rice, grilled chicken, and whatever greens the vendors can put their hands on: a wedge of green cabbage is common, with perhaps a sprig of Thai basil, and beyond that, whatever's handy.
Although I am able to eat spicy food, som tam is one of those things that was sometimes too hot even for me. The thing is that the Thai eat incendiary amounts of chili, and the mortar and pestle is not washed, just rinsed, between batches, so the burning chili juice lives on in its porous surface. I learned to ask for som tam with no chilis added, and still got a fiery dish.
Som tam is sometimes called papaya pok pok in Thai English (Thenglish), this being onomatopoeic with the sound of the pestle knocking on the mortar.
Let's start with the papaya. It's just an unripe papaya, not a special type of papaya, like a green mango. You need to render it into long thin strips for this dish. In Thailand, people either have a special stool-like object which is used to grate papaya and coconut and the like, or they peel the papaya, then hold it in one hand and use a huge sharp knife held in the other to whack the papaya till it falls away in long, thin strips. While these strips make excellent salad, the whole procedure strikes me as risky at best, and I grate mine using the big holes on a box grater or the grating disk of a food processor. Less beautiful, but much safer.
A small papaya will yield about 2-4 c (480-960 ml) grated fruit. For EACH CUP (240 ml) grated papaya, you will need:
Start with the chili and garlic, and grind them together in your giant mortar and pestle. Or just begin to combine everything in a bowl, if that's what you've got. Begin adding and grinding in (or chopping and tossing) the peanuts, beans, and then tomatoes. Add the papaya. Mix together the fish sauce, sugar, and lime juice, then toss the whole salad together. Let sit at least half an hour for the flavours to mix, then serve. Wow, is this tasty!!
This is the classic som tam thai; there is also a version with raw crab (som tam poo), but I would avoid this one for fear of food poisoning. Other variations crop up, of course, Thailand being a country with millions of cooks, but they all build from this basic recipe. Enjoy!
Simulacron3 tells me that s/he had som tam with rice bugs in Laos once, a variation I'm happy to say I've missed. And BlackPawn tells me that in Laos, som tam usually contains no chilis, though it is often served with a tomato based chili sauce.