Thai belongs to the Thai language family, a group of related languages spoken in Thailand, Laos, Burma and northern Vietnam and southern China (by minority ethnic groups). The dialect spoken in Bangkok and its surrounding areas is called Central Thai.
There are three other Tai dialects spoken in Thailand: Southern Thai, Northern Thai, and Northeastern Thai also known as Ee-san. It would actually take someone from Bangkok quite a long time to be able to speak and understand Northern Thai or another dialect of the Tai language family.
Central Thai is the main dialect used by the government, media, and is also used for education. This dialect is spoken throughout the country. People outside of central Thailand usually use their local dialect when at home but switch to Central Thai when they’re in school or doing business.
Thai is a very tonal language. Learning the tones is no easy task and you need a good memory to remember both the phonetic pronunciation and tone of each word. People learning Thai often cut corners and ignore the tones. This isn’t a problem at first because Thais won’t expect perfect pronunciation from a beginner and will usually be able to figure out what you’re trying to say. However, if you want to speak Thai well you should learn the tones, and some words definitely need their tones to be understood, such as “five” (ha) which has a falling tone and “pork” (moo) which needs a rising tone.
Thai words usually have only one syllable. The multi-syllable words in Thai usually concern government, academic subjects, or religion. You may notice a similarity between some high level Thai words and equivalent English words. This arose through a historical connection between Greek and Latin and the ancient languages of central Asia and India (an example is “statistics” being “sa-thee-tee” in Thai). Many modern words are borrowed from English (an example is Thais saying “man” to refer to a masculine male).
Thai like all languages has variations in degree of correctness and formality. “R”, for example, is almost always pronounced “l” informally but on TV, in Thai language classes, and polite situations people will try to pronounce “r” correctly. Vocabulary can also change and there are formal and informal variations for “eat”, “drink”, and many other words. There are also many pronouns that show different degrees of politeness and respect.
One aspect of Thai culture evident from the language is the emphasis on politeness. Using polite forms of language in Thai shouldn’t be thought of as demeaning to the speaker. In its best form the politeness in Thai reflects mutual respect, not a hierarchical social structure. Conversations in Thailand tend to be pleasant and fun. Controversial subjects aren’t brought up and people usually don’t speak sarcastically or abusively. In fact, talking loudly or rudely is taken seriously in Thailand and should be avoided.