Musical theatre uses song, dance, acting and spoken dialogue to tell a story. It's main forms are the European operetta and its American descendant, the musical.
Musical is a stage performance, show or film whose action and dialogue is interspersed with singing and dancing performed by the cast as well as acting. While there is no fixed length for a musical, a traditional musical has around at least fifteen to twenty music numbers (songs). The book or script of a musical refers to the story, character development, and dramatic structure, and is mostly referred to as the libretto (Italian for “little book”). Moments of greatest dramatic intensity in a book musical are often performed in song. Proverbially, "when the emotion becomes too strong for speech, you sing; when it becomes too strong for song, you dance."