Both mean ghost and koreans don’t really care. A ghost is a ghost!
(By this I mean that if you say you saw, or are afraid of, 귀신 they’re most likely not gonna give you 3rd degree to see if it really was a 귀신)
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귀신
ghost with a human figure
(This is just a ghost that has a human-like form. Not necessarily that it looks JUST LIKE a human, it just resembles the human form)
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유령
non-human shaped ghost
Often defined as a ghost with no form. This is a ghost that does not resemble a human.
There are different types of 유령 such as an 악령 (evil spirit) or a 원령 (vengeful spirit. one that harms humans).

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~Shelbi