Astrology, as opposed to astronomy (science that studies everything outside of Earth's atmosphere), is the practice of using the positions of specific constellations to explain or predict human behavior or the belief that the positioning of the stars and planets affect the way events occur on earth. 

A zodiac sign refers to one of 12 specific constellations of the zodiac that the sun passes through. A person's particular sign of the zodiac is the one that the sun was in when they were born. It is a belief in astrology that a person's personality can be predicted using their sign of the zodiac.