So we know there are 4 elements in horoscope, i.e.

Fire, Earth, Air and Water

The elements play an important role in determining the basic character type and the general approach to life.

The elements are also known as the Triplicities. (as there are three signs in an element)

Following best describe the characteristics of the elements (summarised from various source):

Fire signs (Aries, Leo, Sagittarius)
People with a strong emphasis of the fire element are spontaneous and impulsive, they apply their energies wholeheartedly. Their emotional response is quick and they have a lively imagination.

Keywords: Active, masculine, spontaneous, quick, initiating, instinctual, outgoing, vitalizing and energetic.

Earth signs (Taurus, Virgo, Capricorn)
Earthy people react quietly and slowly. They apply themselves with endurance. Emotionally they are deeply rooted and slow to change.

Keywords: Passive, feminine, solid, materialization, physical, stable, slow-moving, practical, sensual and grounded

Air signs (Gemini, Libra, Aquarius)
Airy people are quick and animated. They apply their energies in very diverse ways. They tend to intellectualize their feelings and expectations.

Keywords: Active, masculine, intellectual, ethereal, abstract, ideas, detached, social, conceptual and communicative

Water signs (Cancer, Scorpio, Pisces)
People with a strongly emphasized water element are feeling types and are very sensitive. Their imaginative and emotional lives are deep and rich.

Keywords: Passive, feminine, receptive, sustaining, subconscious, creative, emotions, imaginative, nurturing, introverted and generative