Buddhism like any other religion existing have number of schools of thought. In Thailand the oldest and the most dominant Buddhist school is the Theraveda which means "the Ancient Teaching" or "the Teaching of Elders". Theraveda had been established in India and has been predominant religion in Cambodia, Laos, Burma, Sri Lanka and Thailand.
Learn more about Theraveda Buddhism on the link below.
Link: Thailand news Theraveda Buddhism in Thailand