The Seal of Solomon is a Star of David inside a circle.  It is a symbol of the fusion of opposites, one triangle pointing up, one pointing down, inside a circle which can symbolize the universe, heaven, or the sun.
This is a very old symbol, going back to at least the bronze age, and has been found in many parts of the world.  It is in Mayan temples, in Nepal and Tibet as well as Egypt and Israel.  The seal was said to allow Solomon to command demons and communicate with animals.
    The "opposites" in the symbol have been said to be the mingling of fire and water in the human soul.  In the Kabala, it is seen as the mingling of male and female.  Most importantly though, it symbolizes a flow of energy from above (heaven) to below (earth) and the flow of energy in the reverse direction.  
    Through geometry, in which the Pythagoreans and their followers saw cosmic symbolism, the hexagram and the pentagram became an expression of heaven and its reflection on earth, the divine and its reflection in creation and of the connection between heaven and earth, between the macrocosm and the microcosm, and between spirit and matter.  Geometry has been called the language of the soul.