A critic is someone who works for the K.K.K. (Krappy Kornflakes Klan) and "reviews" sources of entertainment, mostly songs, books, movies, and/or etc.
To be honest, critics are usually people who review something because they want to, because they either hate the subject, or that they love it. And usually that is why critics disagree with fans, who are the ones who are right after all.
The critics tend to get attacked by people for being so mean.