"A tonal language is a language in which pitch is used as a part of speech, changing the meaning of a word" Wise Geek explained tonal languages perfectly!  When  you are using the same word but apply a different tone you "radically" change the meaning of the word. This means that the way you say a word can drastically change the meaning and certainly the direction of a conversation.   Tonal languages are found mostly in Asia, Africa and South America. 

 Chinese is an  infamously difficult language to learn because of the many tones it has. Mandarin Chinese for example has four tones and a toneless tone. The fours tones and toneless tone apparently  exist  because of the small amount of syllables in the Mandarin Chinese language. Cantonese Chinese however has six tones in the language which may represent a different word or expression.  Cantonese.ca noted "linguistic tones are not set at specific, absolute pitches like do or C flat. Instead, they are relative. If you start your voice high-pitched and keep it high like that during the entire syllable, that's the 1 tone. If you then start your voice at a mid-lower pitch and then rise it to the same higher pitch, that's 2. If you start with a low tone of voice and let it drop a bit, that's 4. And so on". There are plenty more examples of Cantonese and Mandarin Chinese tones on these websites Cantonese.ca and People.wku.edu.  Some include audio examples and we encourage you to visit the sites to hear the different tones and to practice them. They may offer a little assistance in your Mandarin or Cantonese studies. 


It's not clear as to why certain regions have multiple tones in a language and why others have none. There are many theories circulating the phenomenon and while linguist have been able to show how tonal languages evolve there is still no concrete explanation for what causes "a society to develop or drop a tonal language". Interestingly enough ancient Greek was once a tonal language,  and the tonal sounds of the language led to the "development of diacritical markings" which enabled written Greek to be understood.  Modern Greek has no tonal element even though it is obviously derived from ancient Greek. Other examples of tonal languages are Thai and Vietnamese which both share unique regional dialects. African languages like KiSwahili and in South America pre Columbian languages like Mayan are tonal.