The Star of David or Magen David (Hebrew: מגן דוד; literally, "David's Shield") is a six pointed shape, formed by two equilateral triangles; one flipped on top of the other symmetrically. According to tradition, the symbol covered King David's battle shield and appeared on King Solomon's ring ("the Seal of Solomon").
Today it is the primary symbol of Judaism and is recognized by nearly all Gentiles as a Jewish symbol. This shift occured in the beginning of the 20th century, when it was extensively used in antisemitic propaganda. To this day, despite its overall positive associations, the symbol is heavily associated in people's minds with the street writings during the Nazi regime. Central to the symbol's spread among Jewish circles was its adoption by Zionist organizations, in sort of defiance against the scorn expressed by antisemitic organizations. Until that time, the Menorah was still the accepted symbol of the Jewish people, and some Ultra-Orthodox sects (such as Neturei Karta) hold that this tradition should be maintained, while rejecting the Star of David.
It has more of a cultural symbolism than a religious one. It is used on the Israeli flag and the shape is often used in jewelry, such as a pendant on a necklace, and worn by Jews similarly to the wearing of a cross by Christians. However, the symbol does not have the religious significance for Jews that the cross has for Christians (representing the martyrdom of Jesus a central event of the faith). From a religious point of view, the Star of David does not represent a specific holiday, ritual, event or have a specific religious meaning. Culturally however, it is a frequently used symbol - often used on interfaith sets of symbols, websites and other places where a "logo" or simple symbol is desired to represent the Jews or Judaism. So, while it may be emotionally meaningful to many, there are no religious rites requiring it or specifying how the star may or should not be used.