This is obviously about the way the music is made up. Is it in two sections? three sections? Are there any repeated sections? Does the music of the beginning come back at the end? Some structures have a "standard", although you'll be hard pressed to find many examples which stick rigidly to the "standard" in music. Such structures are: sonata form, binary form, ternary form, minuet and trio, etc.
Form is structural shape in music. Much of "classical" music shows easily identifiable forms, but nothing is fixed in format, and many variants can be found. Composers write their music as they feel, not to suit descriptions in textbooks! We can only describe the average structures that composers have evolved, and must realise that there will always be diversity. A good composition is one where there is repetition and contrast of a minimal amount of musical material is what helps to make a satisfactory composition. Themes are important, but so too is a contrast of key. Changes of key (especially in Classical times) mark out the significant landmarks in a work.
Before we consider whole movements and how they are structured, we must not forget that melodies and even phrases should be constructed in a careful way. A typical musical sentence (from a simple song, perhaps) might consists of 2 x 2-bar phrases, for example with a cadence in the middle and at the end. But composers try to avoid a feelling of "squareness" (that is, predicatable numbers of phrases and bars) by sometimes extending a bar. This will help to break up a monotonous pattern.
This is a common and popular form with composers of all periods. The simplest way of using this structure is for a theme (either original or openly "borrowed" from another composer) to be followed by several variations. There is never a set of number of these, it is left to the discretion of the composer. Variations are different presentations of the rhythmic, harmonic and melodic structure of that theme.
A rondo is a structure which alternates a recurring theme (which we shall call A) with contrasting episodes (B and C, etc.). The number of episodes can vary, but a typical eighteenth century Rondo might run as follows:
A B A C A + coda or A B A C A D A + coda
As its name suggests, this form is built in two sections, both of which are usually repeated:
Section A : repeat - Section B : repeat
Section B is sometimes longer than section A, and there is often a modulation to a new key towards the ending of section A. The original key will return before the end of the piece.
This is built up in three sections: A B A.
The middle section provides a contrast, and the composer may decide to vary the second A section, but it will be based on the same music.
Sometimes called Passacaglia, this is where a musical figure (the ground) is repeated many times over in the bass part while harmony and melody are added above.
A suite is a collection of dance movements. It was popular from the sixteenth to the eighteenth centuries, although the word is still used occasionally today by composers. Bach is regarded as one of the greatest composers of Suites. The most common movements with most early composers were the Allemande, Courante, Sarabande and Gigue. Between these last two movements were sometimes added a variety of other dances, such as a Gavotte or Polonaise, Bourree, etc. Most of these dance movemetns were in binary form, and the same key was almost always used for each movement, with modulation within movements to provide variety.
This is the most important music form.
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