One of Thailand's most characteristic drinks is Thai iced tea (ชาเย็น chaa yen, lit. "cold tea"). Instantly identifiable thanks to its lurid orange colour, this is the side effect of adding ground tamarind seed (or, these days, artificial colour) during the curing process. The iced tea is always very strong and very sweet, and usually served with a dash of condensed milk; ask for chaa dam yen to skip the milk. A popular variant among locals that is typically sold at markets in the morning is Thai hot tea (ชาร้อน chaa rorn), often served with Chinese-style youtiao (油條) fritters, known in Thai as pathongko (ปาท่องโก๋).
Naam chaa and chaa jiin are weak and full-strength Chinese tea, often served in restaurants for free. Coffee (กาแฟ kaafae) is also widely available, and is usually served with condensed milk and lots of sugar. Ask for kaafae thung to get traditional filtered "bag" coffee instead of instant.
Starbucks is present in Thailand, but for the moment local competitors Black Canyon Coffee and S&P still have the edge in market share. These are the places to look for if you want that triple-mocha latte with hazelnut swirl and are willing to pay 100 baht for the privilege.