Astrology is tied in with the classical elements; earth, air, fire and water. Each sign is supposedly governed by a particular element;
Earth- Taurus, Virgo and Capricorn
Air- Gemini, Libra and Aquarius
Fire- Aries, Leo and Sagittarius
Water- Cancer, Scorpio and Pisces
Each element has various esoteric traits associated with them, and in astrology they also govern personality traits. For example people born under a water sign are predisposed to being emotional, sensitive, artistic and spiritual. Air signs are meant to be more logical, intellectual, discerning etc..
These are fairly general however, I'm sure not every water/fire/air/earth sign is typical to his or her supposed traits, and you'd really need a full birth chart to be more accurate. Any decent book on astrology should be able to tell you more.