Songthaew, covered pick up trucks with rows of seats in the back that transport people along set routes, are a common way to get around in Thailand. You'll see songthaew in almost all urban areas and in lots of beach areas, too. Once you know where they're going and how to pay, they are a convenient way to get around.
Songthaew are part of the informal public transportation system and serve set routes in most urban areas and many beach areas.
In some areas, songthaew have signs indicating their routes, in others passengers go by the color of the truck. The system can be confusing even for locals so tell the driver where you are going before you get on so you don't end up across town.
You can flag a songthaew down on the street. When it stops, hop in the back and grab a seat on one of the benches.
When you want to get out, press the buzzer inside the cab (usually on the ceiling but sometimes on one of the side panels). When the songthaew stops climb out and walk around to the driver's side window to pay for your ride.
Fares are fixed and the driver will tell you how much to pay when you get off. The price of a short ride varies from area to area but expect to pay anywhere from 10 baht for a half mile in Bangkok to 60 baht to get around the island of Samui.
Some songthaew drivers will also take you to your exact destination, but you'll pay extra for it and you can only do it when there are no other passengers.