The four basic substances in nature to allow for life to occur -- Fire, Air, Earth, Water. In Astrology, each element is assigned to three zodiac signs, forming triplicies.
Fire Signs, (Aries, Leo and Sagittarius)
These signs represent energy inspiration, and spirit.
Earth Signs, (Taurus, Virgo, and Capricorn)
These signs have a strong association with physical things.
Air Signs, (Gemini, Libra, and Aquarius
These signs represent the mind, having a focus on verbal relations.
Water Signs, (Cancer, Scorpio and Pisces)
These signs represent the soul and have a strong connection with it
through their sensitive feelings.