A horoscope is a short, broad prediction of someone's future and their character. It's usually based on what period of the year in which they were born (along with where the stars were in the sky). Zodiac is a term that encompasses the periods of the year and star alignment mentioned in the horoscope definition. It provides names to the different periods. Star sign is used to refer to a single period defined by the zodiac. "Aries" for example.