A songthaew is a modified truck that is commonly used as a form of public transportation in most parts of Thailand. Directly translated, the word means "two rows" which refers to the two rows of seats on the truck, facing each other. To board, you just flag one down, like you would a taxi, and then there is a bell you could press when you need to get off. They are somewhat similar to the jeepneys in the Philippines.
I have used the songthaew several times during my visits to Thailand, and they are usually the most affordable way to travel. The challenge, of course, is to make them think that you're a local, which I don't have a problem with so long as I don't open my mouth!
