Opera depends on both words and music. It works best when the music expresses the same feelings as the words. The words of an opera are called the libretto. Sometimes the composer writes the libretto as well as the music.
Opera composers often highlight a particularly important or dramatic moment in the story with an aria or a duet. An aria is a song for one singer. A duet is for two singers. Through the words and music of an aria or a duet, the singers can express the thoughts and feelings of the characters they play.
Sometimes, several people sing different tunes at the same time on stage. This is called an ensemble. The different characters may sing the same words. Or they may sing different words, perhaps expressing disagreement or conflict.
Many operas start with a short introduction by the orchestra. This is called the overture. The orchestra usually plays the overture before the curtain goes up to reveal the stage.