Yahweh, also known as Jehovah and YHWH (or JHWH), is the notional god of the Bible — "notional" because the text fails to present a consistent or coherent picture of the nature, characteristics, abilities or utterances of this deity such as would encourage one to acknowledge that a single real entity is being described. This situation no doubt results from the fact that the Bible is a post-hoc assemblage of texts from a number of different eras and mythic traditions, in some of which the male creator-god is omniscient, invisible, non-physical and all-powerful, while in others he is fallible, visible, requires food, can be heard walking, has limited power, and so on.
The tetragrammaton is the four-letter name of God, i.e. the letters YHWH (Hebrew: יְהׂוָה). The Hebrew writing system didn't originally include vowels and so modern linguists aren't sure of what the vowels of YHWH are.
In the Hebrew Bible, the symbols forming the word YHWH are not pronounced, as the "true name" of God is considered unpronounceable. When reading this word, Jews substitute the Hebrew word "Adonai," which means "Lord."