opera refers to a dramatic art form
originating in europe
in which the emotional content is conveyed to the audience as much through music
both vocal and instrumental
as it is through the lyrics
by contrast
in musical theater an actor's dramatic performance is primary
and the music plays a lesser role
the drama in opera is presented using the primary elements of theater such as scenery
costumes and acting
however, the words of the opera 
or libretto
are sung rather than spoken
the singers are accompanied by a musical ensemble ranging from a small instrumental ensemble to a full symphonic orchestra