As you probably already know, as the Earth orbits the sun each year, the sun is placed in (and appears to move through) different signs of the zodiac. The particular sign of the zodiac in which the sun is located when a person is born, is known as their "sun sign" or "zodiac sign".
According to most astrologers, which sun sign which a person falls under determines many details of that person's character and personality. The strongest effect is supposedly in terms of determining the person's ego, self-control and levels of willpower. Astrologers who believe this, claim that a person's sun sign is strongly predictive of a person's ability to both command authority over, and to influence, other people. Moreover, a person's sun signs are is supposed to be linked to many other aspects of character such as temperament and levels of personal integrity.
Although the characteristics of each sun sign are unique, it is also true that the various sun signs can be grouped together into broad categories, the signs in each category sharing some characteristics. Astrologers do this by dividing sun signs into positive and negative polarities, and further subdividing these into groupings based around the four traditional elements (Air and Fire under positive polarity, and Earth and Water under negative polarity).
It would be wrong however to think that the sun sign completely determines a person's personality. Although astrologers generally believe that a person's sun sign is very important, perhaps the most important of astrological influences, it is nevertheless only one of many factors which affect personality. The positions of the various astrological planets (the actual planets of the solar system as well as the sun and moon) are also said to be important, and these also need to be taken into account when creating a horoscope or performing an astrological analysis.
One other thing to bear in mind is that the start and end dates of the sun signs, although more or less the same each year, do vary slightly. Those people who are born close to, or on, the boundary (known as the "cusp") between two sun signs are often surprised to discover that they do not fall under the sun sign that they think they are. Additionally, people who are born on, or close to, the cusp of two sun signs, may well exhibit characteristics of both applicable sun signs.