Musical theatre (sometimes spelled theater) is a form of theatre combining music, songs, dance, and spoken dialogue. It is closely related to opera, frequently being distinguished by the use of popular music of various forms (and thus usually different instrumentation), the use of unaccompanied dialogue (though some musicals are entirely accompanied, such as Les Misérables, and some operas have spoken dialogue, such as Carmen), and the avoidance of many operatic conventions.
There are three written components of a musical: the music, the lyrics, and the book. The book of a musical refers to the spoken (not sung) lines in the play; however, "book" can also refer to the overall dramatic arc of a show. The music and lyrics together form the score of the musical; the lyrics and book together are often printed as the libretto.
Many familiar musical theatre works have been the basis for successful musical films, or were adapted for television presentations. While some popular television programs have set one single episode in the style of a musical as a play on their usual format (examples include episodes of Ally McBeal, Buffy the Vampire Slayer's episode Once More with Feeling, Oz's Variety, or Space Ghost Coast to Coast's O Coast to Coast!/Boatshow) -- or have suddenly begun singing and dancing in a musical-theatre style during an episode, such as in several episodes of The Simpsons, South Park and Family Guy) -- the television series Cop Rock, which extensively used the musical format, was not a success.
While musical theatre works are performed around the world, they are most frequently produced on Broadway in New York and in the West End in London.
A musical can be anywhere from a few minutes to several hours long; however, most musicals range from two hours to two hours and forty-five minutes. Musicals today are typically presented with one intermission ten to fifteen minutes in length; the first act is almost always somewhat longer than the second act. A musical will usually have around twenty to thirty songs of varying lengths (including reprises and underscoring) interspersed with book (dialogue) scenes. Some musicals, however, are "sung-through" and do not have any spoken dialogue. This can blur the line between musical theatre and opera.
A musical's moments of greatest dramatic intensity 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." A song must be crafted to suit the character (or characters) and their situation within the story. A show usually opens with a song that sets the tone of the musical, introduces some or all of the major characters, and shows the setting of the play. Within the compressed nature of the musical, the writers must develop the characters and the plot.
Music provides an excellent way to express emotion. However, on average, fewer words are sung in a five-minute song than are spoken in a five-minute block of dialogue. Therefore there is less time to develop drama than in a straight play of equivalent length, since a musical may have an hour and a half or more of music in it.