Tone is the use of pitch in language to distinguish words. All languages use intonation to express emphasis, contrast, emotion, or other such nuances, but not every language uses tone to distinguish lexical meaning. When this occurs, tones are phonemes (discrete speech sounds), just like consonants and vowels, and they are occasionally referred to as tonemes.
A slight majority of the languages in the world are tonal. However, most Indo-European languages, which include the majority of the most widely-spoken languages in the world today, are not tonal. The way in which tone is used in a particular language leads to the language being classified either as a tonal language or a pitch accented language.