YHWH (יהוה in the original Hebrew) is a name for the God of Abraham that appears in the Hebrew Bible, or Tanakh and is generally accepted as the name of God in Judaism and the Old Testament. 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" (which is Greek for "a word of four letters"). YHWH is written as it is because Hebrew doesn't usually include vowels. Judaism teaches that saying that name frivolously is blasphemy, even God is sometimes spellt G-d, terms like Ha'shem", lit. "The Name" are used to describe god instead of what are considered sacred names.
Since God never actually tells us His name, a commandment telling one not to take His name in vain is a little confusing.
Meaning and origin
There are several views of what YHWH means, and where the term originated. The most commonly accepted is that the term comes from "Y" (hebrew yodh י) meaning "he", and the Semitic root "HWH", (Hebrew: he wa he היה ) which means either "to be" or "to create" depending on context, mode and inflection, making YHWH "He who is" or alternately "He who creates."
Without vowels, both scholars of the Bible as well as linguists can only guess on the variation of the root HWH, and in context the raw word "YHWH" could also mean "He who builds", "He who lives" (the root "to be" has the variation of "to live" "to breathe" and "to build or bring into existence.") In Exodus 3:14, god states its name is "Ehyeh asher ehyeh" "I am what I am".
A second theory is based on the linguistic evidence that the Semitic tri-root "HWH" indicates things which fall, or a storm. This argument suggests YHWH might be indicating at Storm god. This highly compelling theory blends with the theory that YHWH was an Edomic Storm God who was adopted into the Canaanite culture.
In practical terms, the pronunciation is probably Yahweh or something fairly similar, and this is the usual translation 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." Another common Jewish practice, especially among Orthodox Jews, is to write it as G-d instead of God or to refer to God as "Baruch ha-Shem" ("The Blessed Name" in Hebrew), feeling that even such first-order euphemisms are too sacred to be spoken or written.
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. In most English Bible translations (except for so-called "Sacred Name" editions, which use some form of the Hebrew for most or all names) these letters are translated as The LORD.
The concept of a high god connected to the sky and with severe restrictions on the use of his/its name exists in other religions as well as Judaism. The idea is found in places as diverse as Africa, Asia, both parts of America and and Australia.
—Encyclopaedia Britannica 
|“||His name often is revealed only to initiates, and to speak his name aloud is thought to invite disaster or death; his most frequent title is Father.||”|
It is unclear if this is an indigenous pattern or a reaction to Christian missionaries. 
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.
Though "Jehovah" is generally found in English-speaking Christian groups, it can sometimes be found in English-speaking Jewish sects as well. In the Hebrew alphabet, all letters are consonants, and vowels, if written at all, are written as diacritical marks above and below the letter. In order to remind the reader to do the substitution described above, the word YHWH was therefore traditionally written with the Adonay vowels, yielding "YaHoWaH" , later Romanized into "Jehovah" from being read as written instead of according to tradition. Though almost all Bible scholars agree that Jehovah is therefore incorrect, it remains a somewhat popular euphemism for the name of God.
At least one religious sect, the Jehovah's Witnesses, insist Jehovah is God's real name and insist on its use instead of God. There are also some sects, including many Christian Identity believers, who insist that "Yahweh" is the proper name. Comic writer Larry Gonick, in his Cartoon History of the Universe, poked fun at the controversy by suggesting the proper name of God was "Yahu-Wahu", then spelling it "Yahuwah" throughout the rest of the series.
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) strongly suggests that YHWH 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. The deal also included sacrificing the good stuff—the gold, silver, wine, and animals—to the LORD, i.e. making His priests rich.
Evidences found in the Canaanite city of Ugarit suggests the ancient Israelites practiced a polytheistic North-west Semitic religion, with a creator god, El. Yahweh was associated with El, and became the "national god" of the Hebrews. There is some evidence of a god "YW" who was the Son of El; YW is generally considered to either be YHWH or be some play on words to make a closer association. El (and therefor YHWH) had a wife, incidentally named 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 YHWH 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."
Powers and attributes
Like a particularly badly written fantasy novel with an overpowered protagonist in the vein 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? YHWH is a pretty nasty and smiting guy, who holds grudges. He threw Adam and Eve out of the Garden of Eden for eating from a tree he had told them not to eat from, and decided to curse all their descendants as well. Later this smiting guy became so powerful that a plot device that of, well, they'll think of something eventually, - right now for those who believe Judaism rather than Christianity 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.
YHWH's personal, dominating approach towards ruling was not popular and 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 missing Messiah problem - and failed miserably, ending up with a book full of ten times as many contradictions as before). Of course Jesus kicked YHWH out along with most of his commandments in the early first century. The commandments Jesus said you don't have to obey don't include the ones about homosexuality and bestiality, by the way. Those pesky food laws are obsolete, however.
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).
- Ronald L. Eisenberg. The JPS Guide to Jewish Traditions, p627. Jewish Publication Society of America. 2004.
- High God
Adapted from RationalWiki