There was a time when it referred to anyone who was engaged in the composition, no matter what field of art it may be treated. Because Latin composito means - writing, drafting. The term compositor can be translated as the writer, Creator and attributed it to anything, even to the compilation (song!) elegant bouquets. However, starting from the XVI century the term began to be used in the sense of "composer of the music" (first mentioned, as a rule, "composer"). Now he has only one value.
It is very difficult to become a real composer. For this you need to devote your whole life to music: from childhood to learn to play the piano, have a good knowledge of musical science - the theory of music, harmony, counterpoint, orchestration, history of musical art. You need to study the works of all the major composers of the past and present, to understand the specifics of their work; to examine the features of the sounds of all musical instruments. Only then you can write really good music. Of course, if you have creative talent.
Those who wants to become a composer, studying at music College and Conservatory on the theory and composition faculty.