Answer:
   
   The Star (Shield) of David, also called Magen David, is a relatively
   new Jewish symbol. Supposedly, it represents the shape of King David's
   shield (but there is no rabbinic support for that claim). The symbol
   is very rare in early Jewish literature.
   
   Is there any theological significance to the symbol? Some claim that
   the top triangle strives upward, toward G-d, while the lower triangle
   strives downward, toward the real world. Others note that the
   intertwining represents the inseparable nature of the Jewish people.
   Still others claim the three sides stand for the three types of Jews:
   Kohanim, Levites and Israel. A similar claim could be made for the
   three major movements. However, these theories have little basis in
   historical fact.
   
   What is the history?
   
   Intertwined equilateral triangles is a common symbol in the Middle
   East and North Africa, where it supposedly brings good luck.
   Originally, it was primarily associated with magic or family/community
   insignia. Its geometric symmetry made the symbol popular in many
   cultures. A common claim is that the upward triangle represents female
   sexuality, and the downward triangle represents male sexuality;
   combined, they symbolize unity and harmony. In alchemy, the two
   triangles symbolize "fire" and "water"; together, they represent the
   reconciliation of opposites.
   
   Where did Judaism come into the picture? The earliest known Jewish use
   of the star was as a seal in ancient Palestine (6th century B.C.E.).
   It was next used eight centuries later in a synagogue frieze in
   Capernaum. These may have only been ornamental designs. In the Middle
   Ages, the star appears frequently on churches, but rarely in
   synagogues or on Jewish ritual objects. Also note that Jews of this
   time often wore badges proclaiming their Judaism (similar to those in
   Nazi Germany). However, these badges used a six-pointed badge similar
   to an asterisk, as illustrated in a fifteenth century painting by Nuno
   Goncalves. The menorah served as the primary Jewish symbol, not the
   star.
   
   Some historians have attempted to trace the star back to King David;
   others trace it to Rabbi Akiva and the Bar Kokhba ("son of the star")
   rebellion (135 CE); still others trace it to the kabbalists,
   especially Rabbi Isaac Luria (16th century). However, there is no
   documented evidence of these claim. Instead, evidence suggests that
   the early use of the star was limited to "practical Kabbalah",
   probably dating back to the 6th century. It is connected in legend
   with the "Seal of Solomon," which was a signet ring used by Solomon to
   supposedly control demons and spirits.The original ring was inscribed
   with the Tetragrammaton; but medieval amulets imitating the ring
   substituted the six-pointed star or five-pointed star, often
   accompanied by rampant lions. Hence, the star was called the "Seal of
   Solomon."
   
   Additionally, medieval Jewish texts spoke of a magic shield possessed
   by King David that protected him from his enemies. These texts claim
   the shield was inscribed with the seventy-two letter name of G-d, or
   with Shaddai (Almighty) or angelic names, and was eventually passed
   down to Judah Maccabee. The kabbalist Isaac Arama (15th century)
   claimed that Psalm 67, later known as the "Menorah Psalm", was
   engraved on David's shield in the form of a menorah. Others suggest
   that Isaiah 11:2, enumerating the six aspects of the divine spirit,
   was inscribed on the shield in the outer six triangles of the star. In
   any case, over time, the star replaced this menorah in popular legends
   about David's shield, while the five-pointed pentagram became
   identified with the Seal of Solomon. The star was also widely regarded
   as a messianic symbol, because of its legendary connection with David,
   ancestor of the Messiah. On Sabbath eve, German Jews would light a
   star-shaped brass oil lamp called a Judenstern (Jewish star),
   emblematic of the idea that Shabbat was a foretaste of the Messianic
   Age. The star was also popular among the followers of Shabbatai Tzevi,
   the false messiah of the 17th century, because of its messianic
   associations. Among Jewish mystics and wonderworkers, the star was
   most commonly used as a magical protection against demons, often
   inscribed on the outside of mezuzot and on amulets.
   
   Another use of the star in medieval times was as a Jewish printer's
   mark, especially in Prague and among members of the Jewish Foa family,
   who lived in Italy and Holland. In 1354, Emperor Charles IV of Prague
   granted the Jews of his city the privilege of displaying their own
   flag on state occasions. Their flag displayed a large six-pointed star
   in its center. A similar flag remains to this day in the Altneuschul,
   the oldest synagogue in Prague. From Prague, the star spread to the
   Jewish communities of Moravia and Bohemia, and then eventually to
   Eastern Europe.
   
   The star has achieved its status as the most common and universally
   recognized sign of Judaism and Jewish identity only since 1800. In the
   17th century, it became a popular practice to put Magen Davids on the
   outside of synagogues, to identify them as Jewish houses of worship in
   much the same way that a cross identified a Christian house of
   worship. In Vienna, the Jewish quarter was separated from the
   Christian quarter by a boundary stone inscribed with a hexagram on one
   side and a cross on the other, the first instance of the six-pointed
   star being used to represent Judaism as a whole, rather than an
   individual community.
   
   With Jewish emancipation following the French Revolution, Jews began
   to look for a symbol to represent themselves comparable to the cross
   used by their Christian neighbors. They settled upon the six-pointed
   star, principally because of its heraldic associations. Its geometric
   design and architectural features greatly appealed to synagogue
   architects, most of whom were non-Jews. Ironically, the religious Jews
   of Europe and the Orient, already accustomed to seeing hexagrams on
   kabbalistic amulets, accepted this secularized emblem of the
   enlightened Jews as a legitimate Jewish symbol, even though it had no
   religious content or scriptural basis.
   
   The star gained additional popularity as a symbol of Judaism when it
   was adopted as the emblem of the Zionist movement in 1897. Theodor
   Herzl chose the Star of David because it was so well known and also
   because it had no religious associations. In time, it appeared in the
   center of the flag of the new Jewish state of Israel and has become
   associated with national redemption. The symbol continued to be
   controversial for many years afterward. When the modern state of
   Israel was founded, there was much debate over whether this symbol
   should be used on the flag.
   
   During the Holocaust, the Nazis chose the yellow star as an
   identifying badge required on the garments of all Jews. After the war,
   Jews turned this symbol of humiliation and death into a badge of
   honor.
   
   Nowadays, the Star of David is the most universally recognized symbol
   of the Jewish People.