The ancients were known to describe their physical reality in terms of the four elements of Earth, Air, Fire and Water. By the same token, the characteristics of the twelve zodiac signs can be understood in terms of these four basic ways of experiencing the world.
Everybody has a number of characteristics from each element in their astrological make-up. However, there is usually an emphasis on one element more than another. For example, a person who has their Sun and several important planets in a Fire sign is likely to have a personality which is especially closely related to the archetype of Fire.
The Elemental balance between two people is especially important in assessing their romantic compatibility. In general, Earth and Water signs go together better with each other than with either Air or Fire signs. Conversely, Air and Fire signs go well together, but are not so compatible with either Earth or Water signs. People with a similar elemental balance will have a similar outlook on life, and may be able to empathise well with each other. On the other hand, a person who has a strong emphasis in one element will find a harmonious balance in a relationship with someone whose emphasis is in a different, but compatible element.
The Earth signs Taurus, Capricorn and Virgo are primarily expressions of energy which are practical, physical and rooted in reality. The energy of Taurus especially is focussed on a direct sensory experience of the physical world: we would describe people who have a Taurean emphasis in their chart as being very "down-to-earth".
Because of its physical structure, Earth is also limiting and constricting. People with a lot of planets in Earth signs tend to be a little conservative, and are happiest when they are dealing with a situation that they are familiar with, and which they can experience directly. They are not usually at ease in a situation in which they have to deal with abstractions based on other people's exeriences.
Air is linked to thinking and logic. People with an emphasis in one or more of the air signs, Libra, Gemini and Aquarius are primarily thinkers. Although they are not necessarily emotionally cold people, their relationships with other people tend to be based more on thought and ideas than on pure emotion. This is especially true of Gemini and Aquarius, and less so of Libra.
Air signs have an excellent combination of logical thinking tied to practical reality. They are not as hide-bound by tradition as Earth signs, nor are they as unreal as Water signs. An person with a strong element of Air in their chart will be creative, and will be able to put their creativity into practice.
The element of fire represents positive, outward-going energy. People with an emphasis in one of the Fire signs - Aries, Leo and Sagittarius - work well under their own steam. They push on ahead to get what they want, and are less than sympathetic to anyone who gets in their way.
Fire people have a dramatic quality that helps to energise the people around them. They are warm-hearted, but sometimes lack the sensitivity to understand other people's feelings. While they have the positive energy to cheer up their friends when they are depressed, in practice they find it quite difficult to deal with negativity in others, and consequently may be less helpful in practice.
The three water signs are Cancer, Pisces and Scorpio. People with a large number of planets in these signs are typically very intuitive, and are especially good at sensing other people's thoughts and and feelings. Pisceans in particular are likely to empathize so well with other people that they sometimes find it difficult to hold their own ground.
The water metaphor is very clear: water moves passively to fill a container, in contrast to earth, which holds its own ground, and fire, which creates its own ground.
Like Air, Water represents thinking. However, whereas with Air, there is an emphasis on logical, left-brain type thinking, Water represents holistic, right-brain thinking. In addition, Water signs are typically subjective, rather than objective, and excel in the visual arts rather than in science.