source article: http://www.byzant.com/Mystical/Symbols/Tetragrammaton.aspx
The Tetragrammaton is the holy "four-lettered name" of God. Remembering that Hebrew is read from right to left, this name, yod-heh-vav-heh, is most properly transliterated as YHVH, but sometimes as YHWH (Yahweh) or JHVH (Jehovah). Even though it is the name of God that appears most frequently in the Hebrew Bible, the Tetragrammaton is considered so sacred in Judaism that it isn't spoken aloud and is instead replaced by euphemisms such as Adonai ("Lord") or ha shem ("the name").
The Tetragrammaton is often employed in ceremonial magic, and each of the letters corresponds to one of the Four Worlds of theKabbalah. Our discussion of the Pentagram as Pentagrammaton illustrates the association of the Tetragrammaton with the pentagram.