|For it is so written in|
יהוה (transcription: YHWH) a name for the God of Abraham that appears in the Hebrew Bible, or Tanakh. The original pronunciation is lost to history, partly because those who knew it are gone, partly because the Hebrew Bible contains no vowels, and partly because Conservative and Orthodox Jews would not pronounce the name out of respect. A common rendering of the word is sometimes called the "tetragrammaton" (the Greek for "a word of four letters").
In practical terms, the pronunciation is probably Yahweh, and this is the usual transcription into English. Though, to avoid speaking the name, because that's taboo, when Jews read the Hebrew scriptures aloud in prayer, the four letters are pronounced mainly as Adonai (my lord), but also as Elohim (gods - in plural (singular 'El')) in a few cases. Taking the taboo even further, some Conservative and Orthodox Jews simply call God "HaShem", literally meaning "the name." Most Bible translations represent the four characters as "the Lord" (in small capitals when it stands in for YHWH) and "Lord God" (when it is "YHWH Adonai" or "Adonai Elohim" being translated) with the Jerusalem Bible (and "Sacred Name Bibles", such as the New World Translation used by Jehovah's Witnesses) being notable exceptions.
An earlier, less accurate English rendition of the name is Jehovah, which was formed by a combination of the consonants YHWH and the cantillation marks (vowels) for "Adonai" (that's where the "o" comes from), as the word was written in the Tanakh, so that even the most absent-minded Jew wouldn't pronounce the Sacred Name and be Struck Down in Flame. The "Y" became "J" and "W" became "V" as the sounds corresponding to these letters are absent from Latin. This doggedly incorrect transliteration is still used by certain traditionalist groups and the Jehovah's Witnesses.
Evidence from archaeological excavations (in the Douglas Adamsian tradition of "The Institute of Taking Impossibly Long Times to Find Out What's Painfully Obvious", as even a cursory reading of the millennia-old Tanakh or Old Testament with a right mind - although we're not sure how often that happens - proves that He wasn't the only God, just one that Abraham promised to worship so he could take all of Palestine, put the men to the sword, the towns to the torch, and the women to bed, and sacrifice the good stuff - the gold, silver, wine, and animals - were sacrificed to the LORD, i.e. made his priests rich) particularly in the "Canaanite" city of Ugarit, it's clear that Judaism was originally a variant of polytheistic North-west Semitic religion, in which there was a creator god, El, and his son Yahweh was the "national god" of the Hebrews (Yahweh had a wife, incidentally; her name was Asherah) - amazingly the Mormons got it right for once, and we'll all get our own planet called The Battlestar Galactica, from which humans will evolve in to Cylons and found the Twelve Colonies of Man - and revert to polytheism.
Because Yahweh so obviously made such a lousy cosmic protector, killing his own vassals with plagues, Hellfire and brimstone, his patron kingdoms being ground into dust by the Assyrians and whatnot, he was beefed up by a merger with El, making him not only the particular God of the Hebrews but also THE MOST POWERFUL GOD IN THE COSMOS. It's quite clear through even a cursory reading of the Old Testament (e.g. the First Commandment) that even then the Israelites did not consider Yahweh to be the only god, merely the most important one. Needless to say that he's always mentioned in plural (Elohim), and the single-most repeated phrase in the Old Testament is, "I am the YHWH your Elohim, the El of Abraham, Isaac, and Jacob."
It is speculated that YHWH derives from the root for the Hebrew word "to be" (cf. "I am who I am", Book of Exodus), in the fact that he just is (see Aquinas' Prime Mover, i.e. the Cosmological, argument for God's existence).
Like a particularly badly written fantasy novel with an overpowered protagonist in the vain of Terry Goodkind's Sword of Truth (and to a lesser extent Robert Jordan's Wheel of Time), it's obvious that God increases in power in Genesis, Exodus, Leviticus, Deuteronomy, and Numbers, in ascending order, even though he created mankind when he was weak - and had mortal feet so Adam could hear when he was sneaking up on him after eating the Fruit of Good and Evil (how's that for omnipotent omniscience?) - and eventually becomes so powerful that a plot device - that of, well, they'll think of something eventually, right now it's just an unresolved plot thread - must be brought in to play to explain why we're not all Jews (and why the Messiah is 3500 years late - although a splinter sect of Jews called "Christians" founded by a man named "Paul" elevated a man called "Jesus" to the status of "Son of God" - however without mention of Asherah, more's the loss - to attempt to solve this problem - and failed miserably, ending up with a book full of ten times as many contradictions as before).
YHWH is perceived as male (although he's transcendent - an eunuch? - and "male and female he created them" - an hermaphrodite?), perfectly just (i.e. cruel) and perfectly merciful (i.e. bullshit), and rules usually from Heaven the Seventh (but occasionally coming down for a pint with His creations, usually in a pillar of fire and/or alien spacecraft that kills at least 5,000 Israelites). Sometimes appears as a burning bush. Such is the product of fevered desert nomads' imaginations.
You would have thought they'd have dreamed up an Oasis God instead, but he would have lived in a Champagne Supernova in the Sky (and had better music).