and just a note to what Robert said; It is an International Driving Permit (not a license). It looks almost exactly like that b/s tri-fold cardboard one you get from the US from AAA.
Okay this is FAR off the visa advice topic of the group but this is how getting a thai drivers license works,
There is nothing in the rules that stop people from getting a 2 year temporary thai drivers license (the first one you get) on a 30 day visa exempt entry or a 60 day tourist visa.
IF you have a valid license from your country (and it's written in english) you can take it, your passport, your drivers license from your country, a proof of residence certificate (gotten from the immigration office or from your consulate) and a medical certificate to a Department of Land Transport office. In Bangkok the main one is out by Chatuchak Park.
You usually take 4 aptitude tests (peripheral vision, reaction time, color blindness and depth perception), watch an hour long movie and then you get your 2 year temporary thai drivers license(s) <- that's plural because there's one for a car and one for a motorcycle.
IF you don't have a valid drivers license from your country you need to do the above tests I mentioned, watch the move, sit the computerized multiple choice test and do the practical driving test on their little track.
NOTE: in bangkok you cannot get a certificate of residence from the immigration office unless you are on an extension of stay that does 90 day reporting.