Magen David (Shield of David)
Through our long and often difficult history, we have come to the realization that our only hope is to place our trust in G-d. The Magen David (literally "Shield of David") has six-points, which symbolize that G-d rules over the universe and protects us from all six directions: North, South, East, West, Up and Down. A similar symbolism is found in reading the "Shema." ("Code of Jewish Law" O.C. 61:6)
Rabbi Moshe Feinstein writes that King David used this six-pointed symbol to signify that the Almighty shields him in war. ("Igrot Moshe" - O.C. 3:15)
The Magen David (Shield of David), the six-pointed star made of two triangles, appeared according to Jewish tradition on the shield of King David. According to the same tradition the same symbols appeared also on King Solomon's ring and therefore it is also called the Seal of Solomon.
It is true that six-pointed stars have been misused by the misguided and superstitious people have attempted to abuse them for magical purposes.
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 G-d. The triangle pointing up symbolizes our good deeds, which go up to heaven and activate a flow of goodness back down to the world, symbolized by the triangle pointing down.
Another idea is that a cube, which has six sides, receives form and substance from its solid center. This inner core represents the spiritual dimension, surrounded by the six universal directions. So too, we see this pattern in the six-pointed star with it's hexagonal center. By the way, the same idea applies to Shabbat -- the seventh day which gives balance and perspective to the six weekdays.
Jews were forced to wear a special color in the middle ages, both by Muslim and Christian authorities, as a way of identification. Jews even wore such a badge in Yisrael under the Ottoman Empire. As for the color yellow, presumably this was chosen so that Jews could be recognized from afar.
So whether it is a blue shield waving proudly on our flag, or a yellow shield in a concentration camp, the Shield of David stands as a reminder that for the Jewish people... in G-d we trust.