Intonation is the music of language. It is the way pitches rise and fall, giving meaning to the spoken word. 

All languages are musical.  

What's the music of English?

What's intonation?

Intonation is the use of relative musical pitch to convey meaning in language.

Are there tonal languages?

Yes.  Most languages in the world are tonal.  The relative pitch ("Does the tone of the word rise or fall?") conveys the meaning of what we say.  Truly tonal languages have a difference in pitch to distinguish  every word.  For example, in  Mandarin, a language used in parts of China, the word /ma/ has two meanings. With a rising intonation, it means "mother."  With a falling intonation, it means "horse."    

Is English a tonal language?

In  English, the pitch does not distinguish the meaning of one word from another.  

Then what is English intonation?

English intonation conveys grammatical meaning. 

In multisyllabic words, syllabic stress can distinguish a noun from a verb: (e.g. reCORD vs. REcord. Providing the correct syllabic stress can mean the difference between comprehension and incomprehension (e.g. suPER (incorrect stress) vs. SUper (correct stress).

English intonation conveys the differences between one phrase and another.  It also indicates the difference between a declarative and interrogative utterance.  The intonation of a  declarative statement falls. The intonation of a yes-no question rises at the end of the statement.     

Can the rules of intonation change or evolve?

Yes.  For example, in English there has been a relatively recent change in the tonal discrimination between a question and a sentence.  Many English speakers now end  declarative sentences with a rising intonation. 

So I went to the mall...and I bought some shoes...