And this is the meaning and the origins of the Star of David from the Jewish
Literacy site (http://www.aish.com/literacy/concepts/Star_of_David.asp):

The six points of the Star of David symbolize God's rule over the universe in
all six directions: north, south, east, west, up and down.

Originally, the Hebrew name Magen David -- literally "Shield of David" --
poetically referred to God. It acknowledges that our military hero, King
David, did not win by his own might, but by the support of the Almighty. This
is also alluded to in the third blessing after the Haftorah reading on
Shabbat: "Blessed are you God, Shield of David."

Various other explanations exist on the meaning behind the Star of David.

One idea is that a six-pointed star receives form and substance from its solid
center. This inner core represents the spiritual dimension, surrounded by the
six universal directions. (A similar idea applies to Shabbat -- the seventh
day which gives balance and perspective to the six weekdays.)

In Kabbalah (a mystic form of Judaism), the two triangles represent the
dichotomies inherent in man: good vs. evil, spiritual vs. physical, etc. The
two triangles may also represent the reciprocal relationship between the
Jewish people and God. The triangle pointing "up" symbolizes our good deeds
which go up to heaven, and then activate a flow of goodness back down to the
world, symbolized by the triangle pointing down.

A more practical theory is that during the Bar Kochba rebellion (first
century), a new technology was developed for shields using the inherent
stability of the triangle. Behind the shield were two interlocking triangles,
forming a hexagonal pattern of support points. (Buckminster Fuller showed how
strong triangle-based designs are with his geodesics.)