The Star of David is also referred to as the Magen or Shield of David. It is a six-pointed hexagram that has been used to represent Judaism since the 17th century. It has been used to designate Jews on tombstones in America and in Europe.
The American Jewish Publication Society adopted the star as its symbol in 1873 A.D. This symbol of David 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 they relate to protection and salvation (see Psalm 3:3, 18:35).
Somewhat related to the six-pointed star of David is the five-pointed pentagram known as Solomon's seal (see pictures below). 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 either star because of their association with magic.
An interesting fact related to this symbol tied to David is that the worship of heavenly objects (such as the sun, a planet, the moon, 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 celestial objects (Deuteronomy 4:19, 17:3).
In the 20th century, the star (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.
Israel's flag, adopted on October 28, 1948 (just five months after Israel was established as a country), features the star of David prominently within it. The flag itself resembles a Jewish prayer shawl which is white with blue stripes. While some associate the symbol with the number seven, others believe the twelve exterior lines that make up the symbol represent the twelve tribes of Israel.