In astrology there are many ways to categorize the signs. Polarities, Quality, and Triplicities, aka Elements. Classically the foundation of creation, fall upon four building blocks: Earth, Air, Fire, and Water.
Each of the Elements have three signs, hence the grouping name is Triplicities.
Earth – Taurus, Virgo, Capricorn
Air – Gemini, Libra, Aquarius
Fire – Aries, Leo, Sagittarius
Water – Cancer, Scorpio, Pisces
Also note that each group has one of the each modes, which we will discuss at a later point. But with each mode we see the different concentration of the element. Cardinal being the most concentrated. Fixed being the most unchangeable. Mutable being the most fluid and changeable.
The core values in each of the elemental, and basic approach to life reflect the attributes of the four classic elements.
In case of Fire, it is its approach to life and the desire to live. With Water, experience life through feelings. In Air, it is to cover ground, learn and connect thoughts and ideas. Finally with earth to live through the senses.
The four elemental groups each move forward in life in difference aspects. Covering different areas of the human experience. In an Natal chart, each sign in the same family group lies 120° from each other. Together, with all three creates a trine, or a triangle. This means that the signs or planets, discuss later, flows naturally towards the same goal.