Binary form (or AB Form) is a type of form of music pieces. The word Binary means "in two parts." The word Form is a kind of guideline for composers when they write a piece of music. So, binary form means "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 times and was often used for movements in sonatas for keyboard instruments. Each part of a binary form piece of music are usually repeated.
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 "A" key at the end. Some famous composers of minuets include Johann Sebastian Bach and George Frideric Handel.