YHWHYHWH, or JHVH, is referred to as the tetragrammaton in Judaism and Christianity.
YHWH refers to Yahweh and JHVH refers to Jehovah. Both are commonly used as references to the personal name of the god of Judaism and Christianity, though Jehovah is more popular. However, Jehovah is an artificial word that appeared when the masoretes inserted the vowels of "Adonai" (Lord) between the YHWH, to remember everyone to read "Adonai" instead of Yahweh.
In modern translations of the Bible, the personal name of God is often rendered as "LORD" in small caps.
The personal name of God was so revered by Jews that they often forbade themselves from speaking the word aloud following an interpretation of the Third Commandment, which prohibits the use of God's name in vain. Rabbis often would undergo a ritual purification process before writing the name in their translations.
According to some accounts, each time a Rabbi would encounter God's personal name in the source text, the Rabbi would find a writing utensil that had never been used, and the name would be written with that pen. After writing the name, the pen was never to be used again because it had written the most holy name of God.
The tetragrammaton occurs several times in some of the Dead Sea Scrolls. It is often written in Paleo-Hebrew to set it off from the rest of the text. It is visibly distinguishable from the rest of the text.