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 four Hebrew letters representing the name of the god of Israel in the Hebrew Bible, namely יהוה (YHWH). The Hebrew text does not make clear the correct use of vowel points in the name, and, as such, the correct historical pronunciation it not known.
According to Jewish tradition, the name is sacred and not to be said aloud in casual speech, reading, or even in prayer. The name is instead substituted with one of several titles.
Different vowel points are added to the Tetragrammaton to indicate the title being substituted in, and its pronunciation. For example, the Masoretic Hebrew text has the name written as יְהֹוָה (YHWH with the vowels from "Adonai" added) and as יֱהֹוִה (with the vowels from "Elohim" added) to indicate the reader should substitute the name with "Adonai" or "Elohim", respectively.
Note that the vowels from appropriate titles are added to allow for vocalization of the title in reference to God. They are not the "correct" pronunciation of the name itself, which remains unknown.