iTutor Musics Updates

In most modern music, both melody and chord progression are both present. I understand they both involve changes in pitch. Melody is a single voice where as chords are 3 or more notes simultaneously stacked. What I don't understand is when is melody used and when is chord progression used? Are chords used as background and melody is always the main sound? I'm confused on their role and relationship to each other. Anyone have a summary on these two important elements of music? Thanks!