The term musical form is often loosely used to refer to particular musical genres or styles (Scholes 1977), which may be determined by factors such as harmonic language, typical rhythms, types of musical instrument used as well as historical and geographical origins. In the vocabulary of art-music, however, it has a more extended meaning, referring to the type of "architectural" structure on which the music is built. Scholes (1977) explained musical form as a series of strategies designed to find a successful mean between the opposite extremes of unrelieved repetition and unrelieved alteration.Hmm, not so easy to describe. "Architectural" and "strategies" are both pretty loose metaphors. The idea of finding the right balance between repetition and alteration is a good insight, though. I would say rather the balance between continuity and contrast articulated with melody, harmony and rhythm. But I suppose "articulated" is yet another metaphor! Sometimes it is very hard to talk about music.
Let's clear away a little underbrush. No, form is not the same as genre or style. These words have specific meanings. A genre is a type of piece and may be defined loosely or closely as needed. A minuet is a genre, a dance in 3/4 time, for example. Baroque music, however, is a style involving a great many factors. But form is something different. For example, one feature of the minuet genre is the form AABB. That is, the typical minuet has two parts, each repeated. If the piece is in major, then the first part usually goes to the dominant. This binary dance form is common to a lot of different pieces other than just minuets. The allemande, courante, sarabande and gigue also follow the same general formal plan. Preludes however do not: they have a more amorphous form that varies greatly from prelude to prelude.
The twelve bar blues is a form in which a certain arrangement of tonic, subdominant and dominant chords may be repeated for as long as needed. Sometimes this kind of form is called strophic. In popular songs, this is elaborated by having different music for the verse and chorus. Sometimes another section occurs that contrasts with the verse, usually called the bridge or 'middle eight'. These, like Baroque dances, are sectional forms. You can also have forms that develop continuously without strict sections--often found in Romantic period music. There are a multitude of possibilities based on a number of basic principles.
Now let me rant for a bit. There are forms that are really rather formless. If you take a 12-bar blues progression, or a flamenco progression or any simple harmonic progression and repeat it, varying it each time with different improvisations and you keep doing this until you run out of ideas, then you have had what used to be called a 'jam session' and more power to you. But this lacks a certain element of musical form that I think is important: direction and conclusion. The fact that this kind of music-making does lack this makes it perfect in certain contexts. It is music you can dip in and out of, so it is suitable as background music, not to be listened to closely. It is music that offers free rein to the performers so they have a good time and it doesn't need to be 'composed'. But it is inherently unsatisfactory as 'art-music' because it does not have direction and conclusion. It does not have a planned course that takes you on a musical or aesthetic journey. You don't really go anywhere. And so, it is kind of formless. The next time you hear some music listen for the form or the formlessness. One pretty good clue to whether the music is formed or formless is whether or not notes could be added or taken away with no real effect: if so, it is formless. It is the precise choice of the exact notes and no other ones that can create the sense of direction and conclusion. Two examples:
OK, which one is formy and which one is formless? I've tried to make it difficult for you by choosing these particular pieces. The answer is, the first one is basically formless. Instead of composed form, Keith Jarrett is doing a fantastic job of making the performance make up for the absence of form. My apologies for not finding the whole performance, the clip seems to end in the middle. So my remarks, obviously, apply only to what I hear in this clip. The Beethoven, however, is an excellent example of form with a clear direction and conclusion. I'm not saying, by the way, that one is necessarily superior to the other; I just want to point out the difference.