Why Do Some Jews Spell God "G-d?"?

why do they spell it as "G-d"
do they say God?
do they call him by his name, Jehovah?
Update: Jehovah is God.
In the bible it says to call him Jehovah
to use his name (not in vain)
and when they say it what do they say? Gee Dee?! or God?
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