Intonation is the movement we introduce into our voices in order to convey meaning. We move our voices up or down or some combination of the two over a tone unit, with our intonation forming an intonation pattern or intonation contour over that unit. We use intonation to indicate attitude, grammatical functions or the organisation of discourse.
Example
Try saying these three sentences in the way indicated in brackets and following the intonation contour given in the lines. Notice how the intonation is different in each sentence and how your voice moves across each contour.
He gave you the ticket (said as a statement)

He gave you the ticket (said as a question)

He gave you the ticket (said to show surprise)
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Further reading
Brazil, D. (1997). The Communicative Value of Intonation in English. Cambridge: Cambridge University Press.
Celce-Murcia, M., Brinton, D. and Goodwin, J. (1996). Teaching Pronunciation. Cambridge:Cambridge University Press.
Dalton, C. and Seidlhofer, B. (2004) Pronunciation. Oxford: Oxford University Press.
Halliday, M.A.K., and Greaves, W.S., (2008). Intonation in the Grammar of English. London:Equinox.
Hirst, D.J. & Di Cristo, A. (eds) 1998. Intonation Systems. A Survey of Twenty Languages. Cambridge, Cambridge University Press.
Roach, Peter (2009). English Phonetics and Phonology, 4th edition. Cambridge: Cambridge University Press.
Underhill, A. (2005) Sound Foundations. Oxford: MacMillan.
Wells, J.C. (2006) English Intonation: An Introduction. Cambridge: Cambridge University Press.
https://intonationpatterns.wordpress.com/2012/11/23/summary-by-vivian-cook/