A Tone Language is one where the meaning of a word will change depending on the tone used when it is pronounced. Tone languages include Mandarin Chinese, Vietnamese, Burmese and Thai.
An oft-quoted example is the word ma in Mandarin Chinese which has four different meanings depending on the tone used :
ma + high level tone = mother
ma + high rising tone = hemp
ma + fall rise tone = horse
ma + high falling tone = scold