Hook

Not always a separate structural section - the hook is the single most memorable musical or lyrical phrase in a song. It's what gets stuck in your head. The hook can live in the chorus (the title line), in an instrumental riff (like the opening of 'Smoke on the Water'), or even in a spoken line. A song can have multiple hooks. The difference: the chorus is a section with a specific function; the hook is a moment of maximum catchiness. Producers often say 'hook first, song second'. Because if you don't have a hook, you don't have something people will remember. Everything else serves the hook.