Song forms
Strophic a simple form in which a series of verses are set to identical music
Modified strophic in which some later verses are set to varied music (sometimes only the accompaniment is varied)
Through-composed a song form in which each stanza of the text has substantially new music (there are often, of course, recurring elements, but not with the regularity and symmetry of strophic)
Arias
Ritornello a recurring idea which unifies an instrumental or vocal work. In the Baroque period, it is usually stated at the beginning (in vocal works, by the orchestra, and in concertos, by the full orchestra). It may consist of a number of segments or sub-phrases. In its return appearances, it can be transposed and fragmented (in which only a segment returns). It often comes back intact to end the movement or aria.