In music, form refers to the structure of the composition. It gives a description of the layout of a musical composition and how it is divided into sections.
There are four basic types of musical forms. When a composition is said to be "iterative," phrases are repeated over and over. When it is "reverting," a phrase is repeated after a contrasting one. In a "strophic" composition, a long melody is repeated over and over to different stanzas of a lyric. When it is "progressive," new melodic material is continuously introduced.
To identify the form of a composition, sections can be labeled using letters. For example, a reverting piece has the layout "A-B-A."