The term musical form refers to the underlying structure of a piece of music. This could be a sequence of verses and choruses or a repeating chord pattern or bass line. The most common forms are binary (an A section followed by a B section – e.g., verse and chorus) and ternary (usually an A section followed by a B section, then a return to the A section, which is sometimes an exact repeat, and other times not).
The song Greensleeves is an example of binary form. The A section is played twice (first and second endings) and is then followed by the B section which ends the piece. The whole form can then be repeated with different lyrics for extra verses.