Traditional Thai clothing is a really interesting topic with a somewhat unsuspected history.
In 1950, when H.M. King Bhumibol assumed the throne, along with his wife H.M. Queen Sirikit, much of the traditional Thai dress and costume had been lost to Western style influences that had been introduced over the previous century or so. Queen Sirikit set about establishing a national Thai costume.
By pulling from the few historical photographs that existed, writings and remaining royal fabrics and garments, she worked with French designers to come up with four different looks for women and essentially one look for men that would become recognized as traditional Thai clothing.
The base component of all the variations of traditional dress for women is the paasin. It is a tube of fabric, in more formal cases it is made of silk, that is worn as a skirt. The wearer folds and tucks the fabric to make the skirt tight around the waist with a fold in the front. This garment is not only worn as a part of these formal, national costumes. It is extremely common to see women, especially elderly women, in the villages wearing a paasin with an informal top. You can easily buy cheap cotton versions at the market.
The Thai Chakri outfit is a formal and elegant costume, and likely what most people think of when they think of a traditional Thai costume. It, of course, starts with the paasin, which is typically a yok dork brocade and may even incorporate gold and silver threads into the weft of the weave. The top is another tube of silk and then the look is completed with a sabai, which is the shoulder cloth that is draped around the body, and a gold or silver belt. Gold and silver jewelry is worn as accessories.
The Thai Chilada is a formal outfit that is used for day-time events. I have seen this worn to funerals and cremations. In addition to the paasin, there is a jacket-like top that has five buttons along the front opening.
This style of outfit is registers visually as the most modern. It is clearly a mix of Thai and Western influences and is intended to be worn to events that would usually call for Western-style evening wear. It is a slim-fitting dress made with a brocaded silk and accessorized with either traditional Thai or Western jewelry.
The pakama is not really an article of clothing, but it is a piece of fabric that accompanies most rural men everywhere they go. It is typically a large, rectangular piece of fabric with a plaid pattern woven into it. The men will wear it tied around their waist until its need or use become apparent, such as a head covering in the heat, a hammock, a towel, a sling, a wrap to walk to and from the bathroom with no pants, a bag, a (carefully wrapped) pair of pants or anything else a versatile piece of fabric may be used for.
It is a jacket, typically worn with Western-style pants, though I have seen instances of it being worn with more “Thai-style” pants which are a sewn recreation of the way pants made from a piece of fabric (like the pakama) would look like. The jacket has a Mandarin collar, is slightly tapered, has five buttons and two outside pockets. It may be long-sleeved, short-sleeved or long-sleeved with a sash, depending on the level of formality.
If you are in Thailand and curious about the national dress, I highly encourage you to visit the Queen Sirikit Museum of Textiles in Bangkok.