Maestro (or maestra for women) means "master" or "teacher" in Italian, Spanish and Portuguese. It is used in English to designate a master in an artistic field - usually someone who has gained enough knowledge within that field to be able to teach students successfully, though the term may sometimes be conferred through sheer respect for an artist's works. The term is most commonly used with respect to classical music and opera, most likely due to the heavy influence of Italian operatic traditions in many Western classical and operatic music schools. For instance, composers, orchestra conductors, and music teacher are frequently given this title. The word is less commonly, but sometimes, used in other fine arts traditions such as drawing, painting, and sculpture as well, but there "master", as in Old Master, Old master print etc, is far more common. In this sense it is derived from the rank of "free master" in a guild such as the painters' guild. When not used in a satirical sense
Source:
http://en.wikipedia.org/wiki/Maestro