The 12 astrological signs are associated with one of the four elements: fire, earth, air, and water. These elements demonstrate a way of being or life orientation. Starting with fiery Aries, the zodiac goes from fire to earth to air to water and back to earth. 

Fire: Aries, Leo Sagittarius. Passion,are, energetic, courageous, enthusiastic.

Earth: Taurus, Virgo, Capricorn. Practical, productive, realistic, sensual.

Air: Gemini, Libra, Aquarius. Intellectual, fair, objective, curious. 

Water: Cancer, Scorpio, Pisces. Sensitive, nurturing, empathetic, imaginative. 

Planets in signs in the same element "get" each other. They approach things in similar ways. Even planets that might naturally be at odds with each other, like the Moon and Saturn, tend to "get" each other.  For example, the Moon in Taurus doesn't feel restricted by Saturn in Virgo. They're both Earth signs, both practical and focused on the material and the here and now. The emotional moon is supported by responsible, patient Saturn,  not inhibited. 

Elements have been used in cosmology and spirituality as a way of describing things. They're metaphorical, and they're still a part of our language. An Earthy person, for example, is practical, "down to earth," and the "salt of the Earth." A feisty, spirited person is "fiery," a "firecracker," or "explosive." Elements were used in medicine back in the Middle Ages and Renaissance when all medical doctors were also astrologers and used charts to help diagnose illness and develop remedies by balancing elements.