Musical theatre is a form of theatrical performance that combines songs, spoken dialogue, acting, and dance. The story and emotional content of a musical – humor, pathos, love, anger – are communicated through the words, music, movement and technical aspects of the entertainment as an integrated whole. Although musical theatre overlaps with other theatrical forms like opera and dance, it may be distinguished by the equal importance given to the music as compared with the dialogue, movement and other elements. Since the early 20th century, musical theatre stage works have generally been called, simply, musicals.
Musicals101.com Comprehensive website with reference material on the history of Musicals on stage, film and TV, and other guides on various aspects of the musical genre.
MusicalTheatreAudition.com Comprehensive website offering great tips for successful auditions in the musical theatre.
State University of New York Geneseo Excellent multi-part lecture notes by Melanie Blood on the development of the American Musical.
Wikipedia Very good article discussing the characterisitcs of the Musical theatre form and its history, including a list of famous composers, writers, choreographers and performers.
Wikipedia Long list of Broadway, West End and film Musicals.