Musical form relates to the structure of a given piece, its themes and the relationship between each theme. There are many varieties of form used in music, both popular and classical.
Most popular music is shaped around verses and choruses. Each verse has different words [and sometimes different melody and harmonies as well] while the chorus [generally] remains constant. Forms include VVCVCVC… [V-verse C-chorus], CVCV… CC, and variations thereof.
In binary music, there are just two different themes which appear only once[1], one after the other like this:
AB
Ternary music has two themes, one of which appears twice, like this:
ABA
A rondo [rondeau] works as bit like the Verse Chorus form of popular music today. It is also called Ritornello.[2]
ABACADA
A canon has a repeated melody line which starts at the same pitch, where each entry is delayed by a few notes. A canon must work so that the harmonies match.