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A composer is a musician who is an author of music in any form, including vocal music (for a singer or choir), instrumental music, electronic music, and music which combines multiple forms. A composer may create music in any music genre, including, for example, classical music, musical theatre, blues, folk music, jazz, and popular music. Composers often express their works in a written musical score using musical notation.

Although a musical composition often has a single author, this is not always the case. A work of music can have multiple composers, which often occurs in popular music when a band collaborates to write a song, or in musical theatre, where the songs may be written by one person, the orchestration of the accompaniment parts and writing of the overture is done by an orchestrator, and the words may be written by a third person.

The nature and means of individual variation of the music is varied, depending on the musical culture in the country and time period it was written. For instance, music composed in the Baroque era, particularly in slow tempos, often was written in bare outline, with the expectation that the performer would add improvised ornaments to the melody line during a performance. Such freedom generally diminished in later eras, correlating with the increased use by composers of more detailed scoring in the form of dynamics, articulation et cetera; composers becoming uniformly more explicit in how they wished their music to be interpreted, although how strictly and minutely these are dictated varies from one composer to another. Because of this trend of composers becoming increasingly specific and detailed in their instructions to the performer, a culture eventually developed whereby faithfulness to the composer's written intention came to be highly valued (see, for example, Urtext edition). This musical culture is almost certainly related to the high esteem (bordering on veneration) in which the leading classical composers are often held by performers.

The term "composer" is often used to refer to composers of instrumental music, such as those found in classical, jazz or other forms of art and traditional music. In popular and folk music, the composer is usually called a songwriter, since the music generally takes the form of a song. Since the mid-20th century, the term has expanded to accommodate creators of electroacoustic music, in which composers directly create sonic material in any of the various electronic media, such as reel-to-reel tape and electronic effects units, which may be presented to an audience by replaying a tape or other sound recording, or by having live instrumentalists and singers perform with prerecorded material. This is distinct from a 19th-century conception of instrumental composition, where the work was represented solely by a musical score to be interpreted by performers.

During the Renaissance music era (c. 1400 to 1600) composers tended to focus more on writing songs about secular (non-religious) themes, such as courtly love. Around 1450, the printing press was invented, which made printed sheet music much less expensive and easier to mass-produce (prior to the invention of the printing press, all notated music was hand-copied). The increased availability of sheet music helped to spread composers' musical styles more quickly and across a larger area. By the middle of the 15th century, composers were writing richly polyphonic sacred music, in which different melody lines were interwoven simultaneously. Prominent composers from this era include Guillaume Dufay, Giovanni Pierluigi da Palestrina, Thomas Morley, and Orlande de Lassus. As musical activity shifted from the church to the aristocratic courts, kings, queens and princes competed for the finest composers. Many leading important composers came from the Netherlands, Belgium, and northern France. They are called the Franco-Flemish composers. They held important positions throughout Europe, especially in Italy. Other countries with vibrant musical activity included Germany, England, and Spain.

Composers of music of the Classical Period (1750 to 1830) looked to the art and philosophy of Ancient Greece and Rome, to the ideals of balance, proportion and disciplined expression. Apart from when writing religious works, composers moved towards writing in a lighter, clearer and considerably simpler texture, using instrumental melodies that tended to be almost voicelike and singable. New genres were developed by composers. The main style was homophony, where a prominent melody and a subordinate chordal accompaniment part are clearly distinct.

According to Abbey Philips, "women musicians have had a very difficult time breaking through and getting the credit they deserve." During the Medieval eras, most of the art music was created for liturgical (religious) purposes and due to the views about the roles of women that were held by religious leaders, few women composed this type of music, with the nun Hildegard von Bingen being among the exceptions. Most university textbooks on the history of music discuss almost exclusively the role of male composers. As well, very few works by women composers are part of the standard repertoire of classical music. In the Concise Oxford History of Music, Clara Schumann is the only woman composer who is mentioned. Philips states that "during the 20th century the women who were composing/playing gained far less attention than their male counterparts.