The Dunning-Kruger effect is a cognitive bias where unskilled individuals tend to rate themselves much higher than their actual abilities. Strangely enough, highly skilled individuals tend to do just the opposite; they underestimating their performance abilities.
The effect was named after David Dunning and Justin Kruger of Cornell University after they published a paper in 1999 titled Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments. The paper showed the results of various studies conducted which tested their subjects self assessment in the areas of humor, logical reasoning and grammar. In all three areas, the least competent drastically overrated themselves and to a lesser degree, the highly competent underrated themselves.
So what would explain this strange lack of judgement? According to Dunning and Kruger, "In essence, we argue that the skills that engender competence in a particular domain are often the very same skills necessary to evaluate competence in that domain—one's own or anyone else's.Because of this, incompetent individuals lack what cognitive psychologists variously term metacognition... These terms refer to the ability to know how well one is performing, when one is likely to be accurate in judgment, and when one is likely to be in error. For example, consider the ability to write grammatical English. The skills that enable one to construct a grammatical sentence are the same skills necessary to recognize a grammatical sentence, and thus are the same skills necessary to determine if a grammatical mistake has been made. In short, the same knowledge that underlies the ability to produce correct judgment is also the knowledge that underlies the ability to recognize correct judgment. To lack the former is to be deficient in the latter."
Perhaps a quote attributed to Aristotle would best sum it up. "The more you know, the more you know you don't know."