Tones are present in languages from all over the place: Australia, Africa, South America, North America, Asia (including India). Even some European languages, like Swedish, have some words which use a tonal system to distinguish one from another.
So what's a tone? Well, contour tones have...er...a contour. That is, they don't stay at the same pitch all the way through. Think of the way you say whoops The pitch of whoops rises at the beginning, and then falls away.
Now, if English were a tonal language then this rising and falling might well make whoops mean something quite different from whoops said at a level pitch. It might make the same sort of difference as between the words mother, scold, and horse, to give an example from Chinese of the same word meaning different things if you say it with different tones.
As well as contour tones there are register tones, too. These don't change pitch, they're just high or low. Some languages have a medium pitch as well.
Languages tend to use either only contour tones, or only register tones, but there are African languages which use one system to tell things apart, and the other for actions.
I'll finish with a question to which the answer is yes. It's in Thai, and each word needs to have a different tone for the sentence to make sense:
mǎi mài mâi mái
"Does new silk burn?"
Does that mean there can be no such thing as a tone-deaf Thai-speaker?
I shall have to see if I can find out.
Word To Use Today: tone. This word comes from the Latin word tonus, and before that from the Greek tonos, which means tension as well as tone, from teinein to stretch.