http://www.poetry.org/whatis.htm
Extract
Sound in poetry
Perhaps the most vital element of sound in poetry is rhythm. Often the rhythm of each line is arranged in a particular meter. Different types of meter played key roles in Classical, Early European, Eastern and Modern poetry. In the case of free verse, the rhythm of lines is often organized into looser units of cadence.
Poetry in English and other modern European languages often uses rhyme. Rhyme at the end of lines is the basis of a number of common poetic forms, such as ballads, sonnets and rhyming couplets. However, the use of rhyme is not universal. Much modern poetry, for example, avoids traditional rhyme schemes. Furthermore, Classical Greek and Latin poetry did not use rhyme. In fact, rhyme did not enter European poetry at all until the High Middle Ages, when it was adopted from the Arabic language. The Arabs have always used rhymes extensively, most notably in their long, rhyming qasidas. Some classical poetry forms, such as Venpa of the Tamil language, had rigid grammars (to the point that they could be expressed as a context-free grammar), which ensured a rhythm.
Alliteration played a key role in structuring early Germanic and English forms of poetry (called alliterative verse), akin to the role of rhyme in later European poetry. The alliterative patterns of early Germanic poetry and the rhyme schemes of Modern European poetry alike both include meter as a key part of their structure, which determines when the listener expects instances of rhyme or alliteration to occur. In this sense, both alliteration and rhyme, when used in poetic structures, help to emphasise and define a rhythmic pattern. By contrast, the chief device of Biblical poetry in ancient Hebrew was parallelism, a rhetorical structure in which successive lines reflected each other in grammatical structure, sound structure, notional content, or all three; a verse form that lent itself to antiphonal or call- and-response performance.
How my heart sings 