Jehovah and Yahweh are the two most common ways to transliterate the personal name of God in the Tanach (Hebrew Bible, Old Testament.) Though called in the Bible by such descriptive titles as "God," "Sovereign Lord," "Creator," "Father," "the Almighty," and "the Most High," God's personality and attributes are fully summed up and expressed in this personal name.
Correct Pronunciation of the Divine Name
Jehovah is the most commonly known English pronunciation of the divine name, although "Yahweh" is favored by most scholars. The oldest Hebrew manuscripts present the name in the form of four consonants, commonly called the Tetragrammaton (from Greek te·tra-, meaning "four," and gram'ma, "letter"). These four letters (יהוה) may be transliterated into English as YHWH or JHVH.
The question scholars face is: which vowels are to be combined with those consonants? Vowel points did not come into use in Hebrew until the second half of the first millennium C.E. Furthermore, because of a religious belief that had begun centuries earlier, the vowel pointing found in Hebrew manuscripts does not provide the key for determining which vowels should appear in the divine name. This belief is that the true name of God is too holy to be pronounced out loud, except by the high priest on the highest holy day (Yom Kippur, and is too holy to be fully written. The knowledge of how to actually pronouce it was lost with the high priests.
Yahweh is the most common transliteration of the Hebrew YHWH (יהוה, Y י H ה W ו H ה, note that Hebrew reads from right to left, some web browsers such as MS IE5.5 would display these four letters correctly in bi-directional manner, some web browsers may display the text in the wrong direction), also referred to as the Tetragrammaton. This proper name for God is rendered as LORD or GOD (in small capitals to distinguish it from Adonai, another word translated as "Lord") in most modern translations of the Bible.
The ancient Hebrew alphabet did not include vowels, hence the uncertainty as to the pronunciation of the Tetragrammaton. The Jews never mentioned this name of God, considering it too sacred to be said aloud, and when reading substituted instead one of the other names of God (usually Adonai). When vowel diacritics were introduced, the vowel markings for Adonai accompanied the Tetragrammaton to remind the reader to pronounce Adonai instead of the ineffable name. This led translators in the Middle Ages to inaccurately render the name "Jehovah", and although most scholars believe "Yahweh" to be most near the original pronunciation, "Jehovah" is still more commonly used today.
There is a large debate over the meaning of this name, most agree it is something like "I am the One Who Is". Appropriate reference points in the Old Testament to start an investigation into this name include: Genesis 2:4, Exodus 3:15 (others?).
Historically speaking, the God of the Tanach (aka the Hebrew Bible, or the Old Testament) whether referred to as Yahweh or Jehovah or by some other name, is the same God worshipped by Muslims, Christians, and Jews, and is sometimes thus referred to as the Judeo-Christian God.
See also: The name of God in Judaism