"Romeo and Juliet" is a symphonic poem composed by Pyotr Tchaikovsky. It was heard in the Season One episode "Cartman Gets an Anal Probe".
Background
During the cafeteria scene, Kyle points out Wendy Testaburger from the crowd, whom Stan has a crush on. While the music plays, Stan looks at her, smiles broadly, and hearts appear around his head, referencing the Peanuts animated films. Cartman teases Stan with a song of his own, and Stan denies being in love with her. Wendy walks up to Stan to give him a letter and he barfs out of anxiety.