A Dictionary of Music and Musicians/Maestro
MAESTRO, master. This word is almost exclusively applied to the great classical composers, but occasionally it is used of the very highest class of executive musicians, though even in this case it may be taken as implying an appreciation of their compositions rather than of their performances. It is seldom applied to teachers as such, but refers almost always to composers of note.
Maestro di capella is the exact Italian equivalent to the German term Kapellmeister, or conductor.
Maestro dei putti (master of the boys) is an office which was founded in 1538 (not, as is generally supposed, in the Papacy of Julius II. which was much earlier), and which was first held by Arcadelt. Its duties are to teach singing to the boys of St. Peter's, in Rome, and more or less to superintend the choir arrangements. It thus represents our 'Choirmaster.' [See Arcadelt, vol. i. p. 81.]
Maestro al cembalo is an officer at the Opera, next in importance to the conductor, and occasionally taking his place. His duties consist of superintending the rehearsals of the music, and accompanying at them. This post was held by Handel at Hamburg, when he was quite young [see Handel, vol. i. p. 648], and afterwards by Matheson.