A composer (literally meaning "one who puts together") is a person who creates music, usually in the medium of notation, for interpretation and performance. The level of distinction between composers and other musicians varies, which affects issues such as copyright and the deference given to individual interpretations of a particular piece of music. In the development of European music, the function of composing music initially had no greater importance than the function of performing music. The preservation of individual compositions received little attention, and musicians generally had no qualms about modifying compositions for performance.
Over time, however, the written notation of the composer has come to be treated as strict instructions, from which performers should not deviate without good reason. Performers do, however, play the music and interpret it in a way that is all their own. In fact, in the concerto form, the soloist would often compose and perform a cadenza as a way to express their individual interpretation of the piece.
The term "composer" is often used specifically to mean a composer in the mainstream Western tradition of classical music. In popular and folk music, the composer is typically called a songwriter (since the music generally takes the form of a song).
(from Wikipedia)
