Thailand still has an extensive network of inter-city buses, which is probably how most Thais get around the country. Bus service is available in two classes. Second class is not air conditioned, while first class services have air conditioning, and often much more. A few of the longer routes, such as Bangkok to Chiang Mai, even have a sort of super-first class service with fully reclining seats and other amenities. Bus service is generally inexpensive, but be sure to watch the bags unloaded at intermediate stops, as theft is rather common.
There is bus service from Bangkok to almost every province of Thailand, as well as service between any given province and its close neighbors. You can effectively get anywhere in Thailand by bus. Most big cities have a central bus terminal where buses arrive and depart. In Bangkok, there are three major bus terminals. Buses for the north and northeast, as well as some other parts of Thailand, leave from the massive Mor Chit bus station in the Chatuchak area. Buses for eastern destinations such as Pattaya leave from the Eastern Bus Terminal at Ekamai. The new Southern bus station for destinations as far as Malaysia is on the far west side of Bangkok, in the Taling Chan area.