Star of David
The Star of David is also called the Magen or Shield. It is a six-pointed hexagram used to represent Judaism since the 17th century. It has also been used for hundreds of years on Jewish tombstones in Europe. The American Jewish Publication Society adopted it as a symbol in 1873 A.D. It is frequently seen in synagogues, on vessels, etc. and is an integral part of modern-day Israel's national flag.
The earliest known Jewish source that mentions the shield (Star of David) dates to the twelfth century A.D. Although Jewish rabbinical writings do not discuss this symbol, the Old Testament does refer to shields as related to protection and salvation.
Somewhat related to the six-pointed star of David is the five-pointed pentagram known as Solomon's seal. In ancient magic papyri, the five-point pentagram is often placed on amulets where the various Hebrew names for God (Adonai, El Shaddai, etc.) are written on them in order to (it is believed) ward off diseases. Some Jewish groups reject the use of the pentagram or hexagram because of its association with magic.
An interesting fact is that the worship of heavenly objects (a star like our sun, a planet, etc.) is believed to be man's oldest known form of idolatry. God strictly warned the ancient Israelites that they were NOT to defile themselves by worshipping the sun, moon, stars and other celestial objects (Deuteronomy 4:19, 17:3).
In the 20th century the star of David (usually colored yellow) was used by the Nazis as a method of identifying Jewish people. In Nazi-controlled areas Jews found not wearing the symbol in public could be subjected to severe punishment. The symbol was also used for the design of Israel's flag. The flag, adopted on October 28, 1948 (just five months after Israel was established as a country), resembles a Jewish prayer shawl which is white with blue stripes. While some associate the star with the number seven, others believe the twelve exterior lines that make up the symbol represent the twelve tribes of Israel.