Stanzas provide structure and format within a poem. In many ways they are the equivalent of a paragraph in a prose work. The use of stanzas can make a poem more visually appealing, and give the poem a means of division. Even poems without rhyme or meter will gain structure from the use of stanzas. (PoeWar)

A sestina is usually an unrhymed poem consisting of six stanzas made up of six lines each. The sestina employs word repetition rather than rhyme. The last word of each line in the first stanza is repeated in a different order in the following five stanzas. (Poets’ Graves)

The type of stanza in a poem may be determined by counting the number of lines in the stanza. In English, for example, the most common type of stanza is the quatrain, or four line stanza. (everything2)