When Moses asked God:
Who are you?, He answered:
I am the one that will be (eie asher eie in Hebrew) (Exodus 3:14). This is abreviated to four letters, YHWH, also know as the tetragrammaton. This is from the name Jehova come from (in Hebrew, vowels aren't written down, so that's how you can go from YHWH to YeHoWaH).
Another legend says that God has 100 names, each describing an attribute of Him.