A stanza is a group of lines within a poem; the blank line between stanzas is known as a stanza break. Like lines, there is no set length to a stanza or an insistence that all stanzas within a poem need be the same length. However, there are names for stanzas of certain lengths: two-line stanzas are couplets; three-lines, tercets; four-lines, quatrains. (Rarer terms, like sixains and quatorzains, are very rarely used.) Whether regular or not, the visual effect and, sometimes, the aural effect is one of uniting the sense of the stanza into one group, so poets can either let their sentences fit neatly within these groups, or create flow and tension by enjambing across the stanza breaks.
Louis MacNeice's 'Prayer before Birth' mostly uses a stanza that gradually grows as the unborn baby finds more and more to be scared of, which makes the sudden short stanzas a shock; the content of those stanzas, such as "the man who is beast or thinks he is God", also become scarier. Edwin Morgan's 'Hyena' uses its stanzas to group various aspects of the animal's self-description - his energy, his song, his expectations and so on. Rita Dove, in 'Hattie McDaniel Arrives at the Coconut Grove', follows a similar method of breaking the stanzas, but a phrase continues over the break between the third and fourth stanzas, giving McDaniel's biography and film career the same simultaneous separation and connection in the poem that they had in her life.