Your astrological sign, or sun sign, is calculated from the position the Sun was in when you were born. The heavens are represented by a circle divided into 12 sections, each representing one of the 12 signs of the zodiac.
Each of these 12 signs of the zodiac represents certain qualities and characteristics in personality. Of course, since we are all unique individuals these characteristics can manifest in differently even for members of the same sun sign.
A complete astrological chart takes much more into account than just your sun sign, but understanding the nature of the sun signs is the first step in interpreting a complete astrological chart.