The words of an opera are known as the libretto (literally "small book").
Some composers, notably Wagner, have written their own libretti; others have worked in close collaboration with their librettists, e.g. Mozart with Lorenzo Da Ponte. Traditional opera, often referred to as "number opera", consists of two modes of singing: recitative, the plot-driving passages sung in a style designed to imitate and emphasize the inflections of speech, and aria (an "air" or formal song) in which the characters express their emotions in a more structured melodic style.
Vocal duets, trios and other ensembles often occur, and choruses are used to comment on the action. In some forms of opera, such as singspiel, opéra comique, operetta, and semi-opera, the recitative is mostly replaced by spoken dialogue. Melodic or semi-melodic passages occurring in the midst of, or instead of, recitative, are also referred to as arioso. The terminology of the various kinds of operatic voices is described in detail below.