A group of lines in a poem is called a stanza. When a poem is divided into stanzas, each section is connected to the others through a rhythmic and often thematic pattern. Stanzas are often divided by blank lines.
Stanzas are often described by the number of lines they contain. Couplets, tercets, quatrains, quintains and sestets are stanzas with two through six lines, respectively. An Elizabethan sonnet, for instance, contains three quatrains and a couplet.
Stanzas can be roughly compared to prose paragraphs in that the lines in the group are all connected thoughts. They can also have some fixed rhyming scheme. In formal poetry, the rhythm and rhyme scheme of the first stanza sets the pattern for the rest, unless a stanza is intended to stand out in some way.