The above symbol is also called 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 also been used for hundreds of years on Jewish tombstones in Europe. The star of David symbol was adopted to represent the American JPS in 1873 A.D. It can frequently be 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.
1. O LORD, how my foes have increased! Many are the ones who rise up against me. 2. Many are saying of my soul, "There is no deliverance for him in God." Selah. 3. But You, O LORD, are a SHIELD about me, my glory, and the One Who lifts up my head.
31. For who is God besides the LORD? Or, who is a Rock except our God? 32. The God who girds me with strength and makes my way perfect, 33. Who makes my feet like hinds' feet, and sets me on my high places. 34. He trains my hands for battle, so that my arms can bend a bow of bronze. 35. You have also given me the SHIELD of Your salvation; and Your right hand has held me up, and Your gentleness has made me great. (Psalm 3:3, 18:35, HBFV)
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 related fact is that the worship of things in the heavens, especially stars (i.e. the sun), is considered to likely 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).