A horoscope is a prediction of someone's possible future based on the relative positions of these planets. It can also be referred to as a diagram of the positions of the planets and signs of the zodiac at a particular time and place.
Interpreting a horoscope involves consideration of the positions of the planets and the houses, and the angles made by the planets to each other (aspects). The signs of the zodiac provide the framework in which the planets interact.
By knowing the constellation that the sun was in at the time of birth, one can discover common characteristics to people born under a specific sun sign (also called your birth sign, or star sign). By charting the positions of the planets, and their relationships to each other for a specific time period, one can reasonably predict some things about the future. This is best done by a qualified astrologer who has studied the many factors that go into creating an individual astrology chart.