Dress
The Thais have made the shift to western clothing and today most of the population dresses in this way. In some towns and villages there are people who still dress in more traditional clothing, but this is rare. Most of these clothes are styles particular to a region or time period, but there are also a number of similarities. For women, the dress tended to be colorful and usually loose-fitting on the bottom, but varied on the top as many dresses were tight and rarely were there long sleeves, if the clothes had any sleeves at all. For the men, the style and colors were similar (often reds and yellows, but can come in nearly any color), but historically the men wore little more than loin cloths.
Today many of these traditional styles are still common for formal events and occasions, but few people wear these clothes on a daily basis as western-styled clothing has taken over. Today dresses are still seen, but long pants are more common on both men and women. Shirts are also varied, but tend to be western-styled.
As a visitor to Thailand, there are few dress restrictions in the country. Although few locals wear shorts, there is no discourtesy involved in wearing them as this is a cultural norm, not a religious rule. Of course if you're visiting temples or other sights of particular religious or political importance you may be expected to cover up with long pants and shoulders should be covered at a minimum. Lastly, sunbathing naked or women sunbathing topless is rarely permitted; check with locals before doing so.