God is called by many names in Hebrew. The tetragrammaton (Greek for "word of four letters") is the one name that observant Jews will write down but will not pronounce, considering the word to be too holy for utterance.
Early Christian transliterators pronounced it as Jehovah from at least the 17th century. In the 19th century, the word was retransliterated into Yehweh. The confusion stems from Latin sources, in which the same letter represents both J and Y, and another single letter represents both V and W.
Hebrew is read from right to left. The letters making up the tetragrammaton are (from right to left) Yod, He, Vau, and He. In English, it is commonly written out as YHWH or JHVH.
Occultists based in Judeo-Christian mythology consider the Hebrew names of God (such as Adonai and Elohim) to hold power, and none is more powerful than the tetragrammaton. In occult illustrations, God is most commonly represented by the tetragrammaton.