Short answers, pulled from the story.
Musical form refers to the structure of a musical composition or performance. According to Jeff Todd Titon in Worlds of Music, it encompasses the arrangement of rhythm, melody, and harmony through repetition or variation, and is shaped to create a meaningful experience for the listener.
The first musical idea in a piece is labeled A, and each contrasting section receives the next letter in the alphabet: B, C, D, and so on. Varied returns are marked with superscript numbers, such as A1 or B2, and subdivisions use lowercase letters. The system is described in Craig Wright's textbook Listening to Music.
Musical form operates at three nested levels: passage (how phrases group into verse-like units), piece or movement (the structure of a single self-contained work), and cyclical form (how multiple self-contained pieces are arranged into a large-scale composition such as a symphony or song cycle).
Binary form has two sections of roughly equal length, written as AB or AABB. Ternary form has three parts in which the third repeats or contains the principal idea of the first, represented as ABA. A piece can be binary at one organizational level and ternary at a higher level, as is the case with the Baroque minuet.
Sonata-allegro form is described as the most important principle of musical form from the classical period well into the twentieth century. It consists of an exposition, a development section, and a recapitulation, with optional introduction and coda. It is most commonly used as the form of the first movement in multi-movement works, which is why it is also called first-movement form.
The ground bass, or basso ostinato, is a repeating bass theme over which the upper voices spin polyphonic or contrapuntal threads, or improvise divisions and descants. It was widely used in 17th-century British music and in the passacaglia and chaconne. Percy Scholes, writing in 1977, called it the form par excellence of solo instrumental music.
In 1907, Debussy wrote that "music is not, in essence, a thing that can be cast into a traditional and fixed form. It is made up of colors and rhythms." The statement expressed his conviction that fixed structural forms were not intrinsic to music's nature.