In poetry, rhymes can be much more complex than simply two words that sound the same. When most people think of rhymes, they tend to think of perfect rhyme, or words that are in complete correspondence with one another -- such as pair and mare. Some words, however, rhyme imperfectly; these are called slant rhymes, like the pair "hand" and "gained." Rhymes don't have to sound the same, either. Some pairs of words are considered eye or visual rhymes: they look the same but sound different, as in the case of 'bear' and 'ear'.
Rhyming is common in poetry, perhaps in part because poets have many ways to use rhyme. For example, a poem's lines can each end with rhyming words in what is called a terminal rhyme. But a poem can also contain words that rhyme in the middle and end of each line, as in the line 'I loved you my friend, when I saw you again'. A poem might also begin each line with words that rhyme in what is called initial rhyme.
To talk about rhyme scheme easily, letters are usually used to represent the placement of rhyming words within a poem. For instance, a poem that has four lines, with the first two lines rhyming with each other and last two lines rhyming with each other, would be considered to have an "AABB" rhyme scheme. Rhyme schemes are abundant and vary depending on a poem's form; some poems, like the sonnet, have rigid rhyme schemes; while others are written to suit a poet's particular whims.
Although there's no rule stating that poems must rhyme, some poetic forms follow a traditional rhyme scheme. These include the limerick, Petrarchan sonnet, Shakespearean sonnet, villanelle and pantoum. If a poem doesn't follow each type's particular rhyme scheme, then it cannot technically be considered that type. These rhyme schemes are also tied into the poem's meter, subject and meaning.
To truly understand rhyming poetry, it can be helpful to read examples of several types of poems. A sampling of poets who exercised a mastery of rhymes in their poems includes Emily Dickinson, who used less traditional forms of rhyme; Shakespeare, who composed masterful sonnets; and Edgar Allen Poe, who wrote darkly lyrical poetry in different rhyme schemes.