When people who have never been to Thailand think about Thai food they probably think about things that would be featured on Thai menus abroad like Tom Yum, red and green curries, or perhaps Pad Thai which I have been told isn't even a Thai dish at all but was just popularized by the country.
There is one type of food from a particular region called Isaan that is in the North East of the country where the food has some Laos influence but is very distinct and awesome to the point where the rest of the country also can't get enough of it and it is available everywhere because of how great it is.
This food also tends to be made from very inexpensive ingredients and is very cheap no matter where you order it. Well, I suppose that isnt true. If you ate it at a really expensive resort that is geared towards tourists it would be expensive and sadly, probably not very good.
If you want the genuine article. The REAL Isaan food, you gotta look for the person who at least from a glance, looks least qualified to make it because the opposite is the case.
I can't read a great deal of the Thail language but I can read a few important words that are food items that I really like "Som Tum" is the second thing listed on the bottom there.
You don't need to be able to read Thai to identify these stand though because the person, normally a big boned woman, is probably going to be busy with her giant mortal and pestle before you even approach her. This food is extremely cheap and very popular with basically everyone in the country.
Som Tum is basically spicy papaya salad at its root, but there are a ton of different varieties of it. My experience has been that venturing too far from the original is something that you are bound to enjoy a lot less than the original and I don't really know any foreigners that prefer it over the normal Som Tum which is referred to as "Som Tum Thai." You can throw all sorts of mess in there including a fermented fish something or other that is called "Pla-ra" and it is very stinky. Honestly, until you somehow managed to get bored of the main one I don't see a great deal of reason to venture out.
The process of making this delightful dish is they stick a whole mess of ingredients into the mortar and then mash it all together until all the flavors get all over the place. I couldn't tell you exactly what is in a Som Tum but it goes something like this: Papaya, peanuts, fish sauce, lime, tomato slices, string beans, chilies, and dried tiny shrimps. Personally, I am not really a fan of the dried shrimps or "gung heng" so I will normally ask them to not put them in there. I do LOVE the peanuts though so I will ask for extra of that instead. They don't mind since peanuts are so cheap that they are basically free here while shrimp, as you might guess, cost substantially more.
You can eat it with a fork but I highly recommend that you order it with some sticky rice or "khao neo." This can be used to dip up the spicy and sweet sauce that gives the entire thing it's flavor.
If you are a novice to this you need to be very careful because the small chilis that they use in this process are, at least by my standards, extremely spicy.
They normally use the red ones for the Som Tum and you might thing "oh they are so small, how hot can they be?" Really hot! That's how hot they can be. For a beginner that is not accustomed to South East Asian notions of spicy food it is a good idea to just get one or maybe 2 put into the entire salad. They are going to crush them so the juices will go everywhere. You can always make something spicier in Thailand but once the chili juice is already in there you can't take it out!
For me, the maximum that I can handle and still enjoy what I am eating would be 4 or 5. Since the chilis vary in size I will normally play it safe and just ask for 2. Toy with this at your own risk! one of my favorite Thai dishes can be ruined by someone getting a bit too adventurous with this particular aspect of the process.
So if you are in Thailand you absolutely must try this. It probably wont be directly offered to you but it is literally available everywhere. I have never had a good Som Tum outside of Thailand so while I am sure it does exist somewhere, you kind of have to go to the source in order to get the real deal. I've never met anyone that didn't like Som Tum except a friend of mine who was visiting that can't have anything spicier than BBQ sauce. Thai cuisine was not his forte.