Incredible means "hard to believe," literally "not able to be believed."
Incredulous means "skeptical" or "unbelieving." It refers to a person's response.
The noun form of incredulous is incredulity. The opposite is credulous, or "gullible, believes anything."
Examples: Kim's story was incredible.
Arthur was incredulous as he listened to the story.
See also Credible, Credulous, or Creditable?