Thailand has a unique culture, a blend of various traditions that make it so very different to anywhere else in the world. But a number of things separate the Thais as being Thai - language, religion, monarchy and perspective are the strongest of these.
The first thing that you will notice about the people is that everybody speaks Thai, a language brought with the Thai people from their origins. It is written however in a script that comes from sanskrit, as it was scholars from India that first wrote the language down. Luckily for those who cannot read the script the signposts across Thailand are slowly being converted to ones with roman script. Thai is easy to learn in many respects - it has no tenses, no definite articles and very little grammar. However it is tonal, so many words have a multiple number of meanings depending on how you say them. For the western student, this tonality scares many off, but most of the tonal words can be identified by their context, and the Thais will understand even if you get the tone wrong. Some however are a challenge - the words for 'early' and 'late' for instance are both the same word (sai) but said differently. You can also cause some great hilarity among the Thais when you mistake the word for banana with the word for penis (chuay).
95% of the population of Thailand are buddhist, with the remainder being made up of Muslims and Christians. But it is not quite so clear cut as this, as a lot of Thai people tend to cover their bases and acknowledge more than just buddhism as being spiritually important. You will see spirit houses outside of houses and buildings - an animist tradition that acknowledges the presence of ghosts (all Thais believe) and spirits in the land. There are still animist 'priests' that follow the centuries old traditions of these beliefs, in Isaan you can still have a traditional wedding that goes nowhere near the buddhist temple or the monks. Animist shrines celebrate the life forces of the forests, rivers and trees. It is believed that this animism is the original religion of the Thais prior to the arrival of Buddhism. Buddhism is predominant though, Lord Buddha himself is believed to have walked across Thailand. Most Thai men continue the tradition of entering the priesthood for a time, usually at a turning point or crossroads in their life. This service used to be for a season in the past, between crops, now it is normally a couple of weeks at the least. Women cannot be monks, but some take the white cloths of service and stay in the wat for a time. The monks of Thailand are held in great esteem and are respected. In the morning as dawn is breaking you will see the daily tradition of monks walking around the community and householders filling their bowls with food. This is not begging, the monks do not ask, they give the people an opportunity to generate positive karma by giving.
You cannot avoid the monarchy in Thailand, it is an intrinsic part of what has formed the nation. To a western mind the respect shown for the monarchy can seem to be extreme, but never misunderstand this love and respect for their King and Queen. King Bhumbibol Adulyadej, was the ninth of his line, and his predecessors had more than a token part in the forming of the nation. He has earned his place in history by being a benevolent and intelligent leader, leading the Thais through the great changes that have brought it to a successful country and economy today. The Kings of Thailand resisted colonialism and successfully remained the only country in the region to remain sovereign. The Kings built the irrigation systems that make Thailand one of the world's most successful agricultural producers today. It was the Kings who built the roads and the rail links. It was a King of Thailand that built the first cement plant. The Thais have a picture of the King, Queen and often a favourite historical King in their business premises, homes and public places. Go to see a movie in Thailand and you will have the national anthem played with pictures of the king. Stand straight and respect the tradition, these are not vain autocrats. The country is a constitutional monarchy, so the Thai King is politically only entitled to be a figurehead, but in times of strife like the coups and demonstrations only as far back as 1991-2 the people look to the King for leadership, and what he says is listened to. The present king is an avid jazz fan and plays sax and has composed some pretty good works.
There are a group of qualities which are very much a part of Thai culture that make the Thais who they are. They are best demonstrated using the thai phrases which describe them. The first is Mai Benlai, or 'not to worry'. This is an attitude that can drive a lot of visitors up the wall, but if you can relax and get into it it is very rewarding. No matter how big the calamity, or the problem, the Thai response is invariably 'mai benlai', not to worry. They still fix the problem, or address the calamity, but they do it without a lot of the stress or worry that westerners bring to a situation. The second quality is Krieng Jai, a term without an english translation. However it means that Thais try their best not to rock the boat. They will bend unbelieveably to make you feel comfortable. To a western eye it might look like deference, but it is deeper than that. It is seen most often in the standard Thai response to any question being 'Yes'. Saying No might challenge a situation too much!