Musical form is most often based on the idea of statement and restatement -- introducing one or more themes and repeating them throughout the song, sometimes exactly the same, sometimes changed. Pop music, for instance, relies heavily on this idea. A pop song's chorus (frequently the catchiest part) will be repeated several times throughout, but it will sometimes be slightly altered to incorporate a solo or different vocal melody. Similarly, a pop song's verse could be in complete contrast to the chorus, but it will almost always complement it. This technique of statement and restatement, contrast and complement, is the very base of a song's musical form.
Musical form comes in many varieties and with countless labels, but the most widely recognized are sectional, developmental and variational musical form:
Sectional musical form consists of smaller sections or parts combined in various ways to create a piece of music. Binary musical form (ABAB) is a sectional musical form using repetition, with A and B representing two different parts; pop music tends to rely heavily on binary musical form.
Developmental musical form is created by introducing one or more themes (as opposed to individual parts) and presenting or combining them in different ways throughout the piece; the most common developmental musical form is the symphonic sonata-allegro musical form.
Variational musical form combines traits of both the sectional and developmental musical forms. Like sectional, it relies on smaller sections or parts combined to create a whole. Variational musical form, however, gives a different treatment to each section before repeating it; where developmental musical form changes the presentation of the material, variational musical form changes the material. This can be done by altering melody, harmony or even instrumentation. Jazz improvisation is the most well-known example of variational musical form.