Binary form (or AB Form) is a type of form of music pieces. The term Binary means "in two parts", while Form in music acts as a kind of guideline for composers to design or shape a piece of music. When "binary" and "form" join together, you will get a basic definition of "An outline in two parts". A musical piece in binary form is a musical piece with two sections. Binary form was popular in the Baroque period and often used to structure movements from sonatas for keyboard instruments.
Generally, a piece in binary form will compose of two parts, which are roughly the same in duration and tones. There are only a few differences between the first part(A) and the second part (B). The second section is longer than the first section and both often repeat.
Binary form was most commonly used in the Baroque era for short dances called minuets. The minuet's two parts are usually labeled as "A" and "B." In the A section of a minuet, the exposition is stated. This section almost always is repeated a couple times. The beginning part of the B section develops a slightly new musical idea, usually going to the dominant key, to add variation. Then it returns to the home key for the finish. Some famous composers of minuets include Johann Sebastian Bach and George Frideric Handel.