The libretto is the script of the play outside of the musical numbers. Also called the "book," it dictates the play's plot and provides the characters' nonmusical dialog. John Kenrick of Musicals 101 explains that an effective libretto maintains the clarity of the storyline and provides the timing that allows the characters to move in and out of songs in a seamless manner. Musicals with great songs but weak books often fail, as they do not fully develop the characters or tell interesting stories.
The score is the musical component of the production. Theatrical Musical identifies the score as a combination of melodies that establishes the tone of the story. The score is more than just the songs that are inserted between dialog. It also includes the overture, which is the musical piece that introduces the production, and the music that is played between scenes. It can even include specific sound effects that are created by instruments in the orchestra.
Lyrics are the words the characters sing. In Broadway musicals, lyrics can be the words expressing one character's thoughts and emotions in song, or they can be dialog set to music in which two or more characters actually sing a conversation. Lyrics in a musical go beyond mere poetry. They also define the characters' personalities and move the libretto's plot forward.
Choreography is the dancing presented in a Broadway musical. Dance Advantage notes that choreography for a musical is much different than that for a dance recital. The choreographer must read the libretto to understand the plot and study the score to get a sense of the mood. Dance is often used in musicals to push the story forward through physical action. Choreography in "West Side Story" depicts a climactic rumble between two rival street gangs and even depicts a character's death.