Verse is text that is felt to be language of particular seriousness, intended to be remembered and repeated. The verse text makes for this object by dividing language into defined segments that may easily be grasped by the mind. Besides the divisions within the language as a whole--into sentences, parts of sentences, groups of sentences, and so on--there is here an additional division, into correlatable and commensurate segments, each of which is called a verse line. The Greek word for verse, stichos, means 'row', and its Latin synonym versus (from which we get 'versification') means 'turn', return to the beginning of the row'; and 'prose' in Latin indicates language 'which goes straight on' without any turns. It is a general requirement that the boundaries of these segments be laid down for all readers (or listeners) by extra-linguistic means: in written poetry usually through graphics (division into lines), and in oral poetry usually through a melody or a standard intonation that resembles a melody. When the text is being perceived, the mind takes account of the size of the segments and anticipates their boundaries. The confirmation or denial of this anticipation is perceived as an artistic effect.
What do we mean when we say 'a general requirement'? Prose is also divided into stretches of language, but their segmentation is more arbitrary. The first sentence of James Joyce Ulysses can be read with equal justification both this way: 'Stately, plump Buck Mulligan / came from the stairhead, / bearing a bowl of lather / on which a mirror and a razor / lay crossed'--and this way: 'Stately, plump / Buck Mulligan / came from the stairhead, /bearing a bowl / of lather / on which a mirror / and a razor / lay crossed'. But if Joyce had written down his text like this: