The term "musical form" refers to the underlying structure of a piece of music. This could be a sequence of verses and choruses, or an A section and a contrasting B section, 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, sometimes an exact repeat, 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.
The chromatic scale can be built on any note. Here is the D chromatic scale:
Here is a piece by Johann Sebastian Bach which uses ternary form (A B A)