Hardly any Thai people themselves use these services, and you will only see Westerners and Asian people from outside Thailand. Strangely enough, though traditional massage has nothing to do with sex, very few females seem to be interested in getting the service. To open a massage venue, the masseuses need to be certified (at least some will be). Many girls will claim to have learned their profession at Wat Pho, although that is very doubtful. There is one easy way to figure out whether someone has been trained properly. At the beginning of the massage (which starts at the legs), the masseuse will pressure your femoral arteries both sides for about 15-30 seconds. If done properly, you will feel your arteries pumping vigorously against the girl's wrist. If this does not happen, it means they are not capable to find the proper location, and just pressure in the vicinity.
Thai traditional massage is quite different from massage you will mostly get in expensive massage places in the West. It is based on 'pressure points', and a lot of stretching. If you are not used to it, some treatments might be painful. They will ask you from time to time how it all feels, but if you feel pain, you should tell them. Once you have done it a few times, it will all be more comfortable, since you will be used to the stretching of your tendons and ligaments.
Unfortunately a lot of time is spent on the legs (probalby dating from the time most people did a lot of walking). Then the arms are massaged. The last and most pleasurable part of the massage will concentrate on your back and neck. Lightly built girls will walk all over you and massage you with their heels. When they make you sit up, they will probably want to crack your spine. If you are uncomfortable with this (I am), you should tell them. Body language will usually be understood, and some of the girls will ask you whether they can do it. Never heart about any problem with this, but I am apprehensive about it. After all, the masseuses are not exactly chiropractors.
Whether the girls are fully qualified and certifies or not, overall it is a good experience, which I would recommend to all visitors to Thailand.

Over the past few years, most massage places, will now also offer oil massage and reflexology. Reflexology could simple be called foot massage. For a full hour you will get attention concentrated solely on your feet and lower legs. So if you had a long day of sightseeing, it may look very appealing.
It is a different kind of massage alltogether. You have to take off all of your clothes (for traditional massage, you are given a pijama outfit). There may be some tradional massage added to it, but oil massage mostly consists of gently rubbing the skin all over your body. As it turns out, as you are male and in the nude, the masseuse girl will at the end most likely ask you whether you need some massage of your private parts. She will usually be disappointed if you decline this offer.