FORMS/ STYLES
*Whereas most popular styles lend themselves to the song form, classical music has been noted for its development of highly sophisticated forms of instrumental music: these include the concerto, symphony, sonata, suite, étude, symphonic poem, opera, and others.
Classical composers often aspire to imbue their music with a very complex relationship between its affective (emotional) content and the intellectual means by which it is achieved. Many of the most esteemed works of classical music make use of musical development, the process by which a musical idea or motif is repeated in different contexts or in altered form. The sonata form and fugue employ rigorous forms of musical development.
http://en.wikipedia.org/wiki/Classical_music