There are several aspects of etiquette and custom, but as in most countries, the importance of these does vary considerably in different cultures within the country. (What is considered acceptable by the majority on the tourist beaches, or in modern Bangkok, and what is acceptable in conservative village life may differ). However, to be on the safe side as regards offending anyone, it is best not to touch the top of anyone's head (sacred) or show the sole of the foot or point with the foot (offensive). As 'livingabroad' also says, Thais don't like displays of temper. Public displays of affection like kissing may also cause some embarressment to some people. The one thing that seems universal, is the taking off of shoes when entering temples (absolute), or people's houses, and Thais will also remove shoes on entering hotel rooms. Importantly regarding religious respect, avoid wearing revealing clothes particularly around temples and the like, and respect Buddha statues and also images of the king. Women should not touch Buddhist monks in greeting.
However, I must stress that Thais are tolerant and they fully understand Western ignorance of their culture and therefore tend to be very accepting of genuine minor indiscretions. I've been to Thailand 5 times and had no problems.
One last note, although Thais of course accept handshakes etc as a Western greeting, the traditional Thai greeting of the 'wai' - the prayer-like pressing of hands together - is much appreciated. I wish it existed in Western society - so much more beautiful, graceful and polite than a handshake!
(If the question indicates you're going there, have a great time. I've been there 5 times, and it's an enchanting country).