It's a songthaew, and it is a cheap, fun way to get around the cities of Thailand.
The word songthaew, (pronounced like "song" "tau") means "two rows" or "two benches" depending on who you talk to, and they are shared taxis that will take you anywhere you want to go, as long as they are already going that direction.
If you don't speak Thai, ask someone to write down your destination in Thai and show it to the driver. Don't feel snubbed if he says no, he's just not going that direction. But if you get the nod, ask how many baht (it won't be much) and jump in the back.
The ride can best be described as "like a roller coaster, baby" so hang on.
The order of pick ups and drop offs is entirely in the driver's head, so keep an eye out for your destination.
In some cases the driver will tell you when you are at your destination, sometimes not. If you miss your stop, I say get off at the next interesting stop and enjoy that place.