The villancicos are strophic, but they tend not to extend to the many strophes of narration that were typical of the romance. A standard form for a villancico strophe is ABBA, in which the first A is called the estribillo (refrain), the Bs represent the statement of the music for the copla (stanza) twice (as copla and mudanza), and the return of the estribillo as the vuelta.
Source: wiktionary