I believe that revenant and ghost are the same, although I can't remember coming across the former outside of fantasy games or pre-20th century literature. The word "spook" almost always points to someone who works for a national intelligence organization.
This Yahoo Answers page might help with distinguishing between spooky things: https://answers.yahoo.com/question/index?qid=20110919075224AAgGHbB
February 15, 2016
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