The main religion in Thailand is Theravada Buddhism, but there are also Muslims, Sikhs, Hindus, Christians and other religions in Thailand. Buddhist teachings have had a large effect on the culture and attitude of people in Thailand. There are stunning Buddhist temples which are very interesting to visit. Thai culture is also affected by the different ethnic groups living in Thailand, such as the different hill tribes from China, Tibet, Laos and Myanmar, and the sea gypsies in the south. Thailand is very interesting historically and has some very impressive ancient monuments.
As it is a Buddhist country nudity or topless sun bathing is frowned upon. This may seem odd given the Thais' tolerance of prostitution but Thais are very modest. If washing in a public place, keep yourself covered with a sarong. Outside tourist resorts men should not wander round without a shirt on and women should not wear sleeveless tops or short skirts/shorts. Don't point your feet at anyone or any sacred image or object - the feet are the lowest part of the body and are considered the lowest part of the body spiritually as well.
The Thais revere their monarch, so do not insult the King or be negative about monarchy. Stand when national anthem is played, for instance in the cinema at the start of a film, or at train stations at certain times of day.
It is a good idea to try and learn a little Thai. Being appropriately dressed and able to at least say 'Hello' (sawat dii kha/krap - see Thailand language for more details) in Thai will get you a much better reception from the Thais.
Thais do not express emotions, particularly anger or sadness. Thailand is known as 'the land of smiles' and they will always smile or laugh in response to difficult situations which Westerners may find frustrating. This makes Thais hard to read, and it takes a long time for Westerners to really become assimilated into the Thai culture and understand it. People may tell you what they think you want to hear rather than risk upsetting you. This isn't done with the intention of lying or being malicious. Thailand is a very laid-back country and Thais aren't the best time-keepers so don't get expect buses to run on time, or people to turn up when they say they will.
Thais don't have the same concept of personal comments that we do and they will happily tell you that you are fat, or you smell without any embarrassment. They also do not like facial hair very much so if you have a large beard, this may attract comment.