Form
Form is defined as the underlying structure of the music over time and can refer to a section of the performance or describe the piece in its entirety. Some types of compositions, particularly in classical music, are identified by this adherence to a form (e.g., sonata). Popular music exhibits a tendency toward forms based on lyrical content, such that you have verse sections, in which the lyrics change, and refrain or chorus sections, in which the lyrics are the same each time they are played. The latter typically include the melodic hook.