The name Star of David, derived from the Hebrew "Magen David" (literally "Shield of David"), hints that there would be a direct connection between the star and (King) David.
A legend states that David would have worn a Star of David on his shield, whenever he went into battle. Not a single shred of evidence was ever found for this however.
The Star of David is a hexa-angular star, also called hexagram. This hexagram was used during the ancient history as a magical sign by among others by the Greek, Romans and the Babylonians, and taken over later by among others the Jews. In the Middle ages the Jews used the hexagram specifically to the following three ends:

• as a decoration, and in particular as a graphical picture of a rose with six half round leaves;
• as a magical symbol. Thus pregnant women used the image of an hexagram as protection for the unborn child; others used it as a deterrant against demons;
• as an astrological symbol, and in particular as a graphical picture of the sun or a big star.

This was as far as it is known the first time in history that Jews had to "mark" themselves. In the 13th until the 18th century AD this repeated itself: on instigation of the church and the government Jews (as well as Arabs) in many European (!) countries were forced to wear certain identifying marks (in some countries, among which The Netherlands, again a yellow spot) on their clothes. Again this had the reason to prevent mixing with other religions, in this case the Christians. Only under the influence of the ideas of the Enlightening, people's rights and the French Revolution (18th century) people came to the conclusion, that Jews could be treated as equal citizens as well and the identifying marks were abolished again. The Netherlands in this by the way were fast at this in Europe; in 1619 already in our country the obligation to wear an outer distinguishing mark disappeared.
Simultaneously with the yellow spot with which Jews were 'branded' in the Middle Ages, the hexagram, which more and more became known under the name "Star of David", developed itself to a specifically Jewish symbol. More and more Jews started to use it, e.g. in synagogues, on drawings and on stamps. In 1354 or 1357 (the exact year is not known) the Emperor Karel IV granted the right to carry their own flag to the Jewish community in Prague. On this flag made of red and gold the Star of David is depicted, with in the middle of this star a Jew hat. Around this time the Star of David started to serve as a "decoration" on Jewish tombstones.
Striking is that the Star of David was depicted in 1656 together with a cross on a borderstone in Vienna, which outlined the Jewish and Christian parts of the city. Later on you could find the Star of David in various Jewish communities situated in the Middle of Europe.
In the 19th century the Star of David had become the most occurring and generally recognized Jewish symbol. Also non-Jews then started to acknowledge the Star of David as a Jewish symbol. Striking is that the Star of David especially in those countries with a great tolerance towards Jews, developed itself as a Jewish symbol .

 
A negative Star of David
Through the centuries Jews dreamt of returning to the promised land (Palestine), but ever since the year 70 AD they hadn't tried seriously any more to realize this. At the end of the 19th century this changed however. Under the influence of a growing anti-semitism and violent pogroms in Russia, Romania, France, Germany and other countries and an increasing nationalism throughout Europe, a movement originated within Judaism which had the objective of creating a Jew state in Palestine, which could take care of Jews spread out all over the world. This movement, known under the name of "Zionisme, consisted of several clubs, each of which for itself used different forms of the Star of David as a distinguishing mark (a.o. a Star of David with in the middle the picture of a lion and around it seven stars, one with in the middle of it the word "Zion" and the current Star of David). Eventually the Star of David developed into a Zionistic sign, which called (and still calls) upon Jews to return to the land of the fathers. By the way the Zionist movement in those years wasn't too big yet, partly because Jews in many countries (among which The Netherlands) had become fairly integrated. In the thirties of this century however this changed drastically by anti-semitical and fascist political movements emerging all over Europe, which caused an ever increasing discrimination of the Jew. These movements started to abuse the Star of David as an anti-semitical sign; the Star of David symbolized in this the "Jewish danger". So "the Jew" was often depicted in pamphlets as a capitalist with a fat belly and a cigar, who keeps people attached to strings and who controls the factories. At that this caricature wore a Star of David, through which this Jewish symbol got more and more of a negative ring with people.

In 1933 Hitler decided that Jews in Germany had to wear a brand, and in particular with a Jew Star (the insulting name for the Star of David). On the colour the nazi's didn't have to think very long. This had to be yellow, because this colour had been common in a negative and stigmatizing way to Jews for centuries already. The nazi's even spoke of "jew yellow". On May 2nd, 1942 the yellow Jew star was "taken into use" in the Netherlands. During the years 1933 to 1945 the Star of David therefore fulfilled a negative (the yellow Jew star) as a positive (the Zionistic Star of David) function. The Zionistic Star of David at that most of the times was blue on a white background, inspired by the Tallit, the blue-white Jewish prayer cloak.

As on May 14th, 1948 the state Israel was established, the blue Star of David on the white background became the -logical- national flag. The Star of David now is the symbol of a proud nation, a sign of hope for every Jew, who seeks protection in an own country. A positive Star of David.