The four elements
Empedocles, a fifth century BCE Greek philosopher, originated the terms Fire, Earth, Air, and Water. He explained the nature of the universe as an interaction of two opposing principles called love and strife manipulating the four elements. He stated that these four elements were all equal, of the same age, that each rules its own province, and each possesses its own individual character. Different mixtures of these elements produced the different natures of things. Empedocles said that those who have near equal proportions of the four elements are more intelligent and have the most exact perceptions.
Each sign is associated one of the classical elements (water, fire, earth and air.) Fire and Air signs are considered positive or extrovert, masculine signs; while Water and Earth signs are considered negative, introvert, feminine signs.
Definition courtesy Wikipedia.