- credulous: (adj.) Tending to believe something too quickly, with little or no proof. Also comes in noun form (credulity, credulousness) and adverb form (credulously).
- credible: Believable, and generally considered reliable.
- incredulous: Unwilling or unable to believe.
- incredible: Unbelievable or too improbably to be believed, though when most people use the word, they're rarely indicating that they actually do not believe. More often than not, incredible is used as a synonym for outstanding or surprisingly good instead of not possible.
Think of the old saying, "If it sounds too good to be true, it probably is." This means that you should be incredulous about something that is incredible (in its basic sense) because it doesn't come from a credible source.
The word credulous comes up often when talking about blog posts and news reports (too often, it seems, when talking about science reporting at the BBC). Reporters who are too credulous take something they hear and then report it as news without taking the time to check the facts, so you can't really trust their reporting. So remember: If a reporter is too credulous (think cred-u-LESS), then you can rely on his reporting LESS.
On the other hand, if a report comes from a credible source, it's something you can swallow — metaphorically — which makes it edible — also metaphorically.
So, if a reporter is credible, his news is edible. If he's credulous, trust him less.