AB (Binary) Form
AB or Binary Form is a form of music which is made up of two clear different sections of music labelled, Part A and Part B.
Think of this as a cheese on toast sandwich.
The two sections generally share a similar harmonic direction and are roughly equal in length.
The A section can be repeated as long as it is repeated before moving onto the B section. The B section can also be repeated at the end.
Therefore AB or Binary form can be recognized as AB or AABB.
The piece Greensleeves is a good example of a piece in binary form: