A concerto is a musical composition, often in more than one movement, in which one or more solo instruments play along an orchestra (or some other comparatively big instrumental force). The etimology of the word suggests that the soloist(s) and the ensemble develop the piece by presenting contrasting episodes where they play independently, in cooperation, and in different levels of subordination. The concerto, as understood today, arose in the Baroque era with the 'concerto grosso', a musical form where a small group of players contrasted against a bigger orchestra. While the concerto grosso eventually declined, the solo concerto remains one of the most popular musical forms in Western classical music.
Having successfully survived through many the periods of Western classical music history, the Concerto underwent many formal changes. It is conventional, if only as a basic definition, to say that the concerto is usually a multi movement work, of which the first movement is roughly structured around the allegro-sonata form (even if Mozart himself treated his concertos with notable freedom in terms of formal development). The second movement of a concerto, as in the sonata/symphonic tradition, is usually slow and contrasts with the first one. It can be in an abridged sonata form, a romance, a set of theme and variations, etc. The third movement will traditionally return to the home key in lively tempo, usually in the form of a dance, rondo, etc.