Sonata form is a very rigid method of musical composition; it can refer to either an entire piece of music or the first movement of a symphony. Though sonata form is an integral part of a symphony, sonata form as an entire piece of music tends to be a bit more complicated as more attention and time is given to each individual part within the intricate composition.
Sonata form is generally divided into five parts: introduction, exposition, development, recapitulation and coda. The introduction within sonata form is generally the shortest part of the piece, at times omitted completely if it isn't completely necessary. A sonata form introduction is often dynamic-building, a slower part that increases with weight as it moves forward, pushing the song toward the exposition -- the most important part.
The sonata form exposition is the meat of the song, the area in which the major thematic subjects are introduced. The exposition itself is divded into a series of subject groups. The first subject group in sonata form introduces a theme, followed by a brief transition used to switch keys. The second subject group brings altered melodies into the sonata form in a different key than the first group; it is primarily used to change the piece's mood. It is followed by the sonata form codetta, which revisits various themes within the exposition and signals the end of the part.
Sonata form development is exactly that: development of the subject found in the exposition. In order to make the sonata form development work effectively, however, the subjects have to be very altered, sometimes introducing one or two elements completely new to the song. It is followed by the sonata form recapitulation, which revisits the exposition. It is divided into the same groupings as the original exposition with the parts unchanged -- except for the transitional key change, which is usually abandoned.
The final part of the sonata form is the coda, a part that essentially repeats the end of the song. The sonata form coda can, of course, be altered, but it is rarely so altered as to become unrecognizable.
