My favorite page on WIkipedia is the List of cognitive biases, which I consider fundamental reading material for human life. Cognitive biases are the ways in which humans systematically, reliably come to flawed conclusions about the world around them. They're a lot like logical fallacies, but they have more to do with problems of perception and mental organization than flaws in logic or comprehension of causality, although sometimes the categories overlap. The Dunning-Kruger effect is one of my favorite biases, by which I mean it's among those I spend the most time thinking about and most consciously try to compensate for.
It refers to the tendency for people who are less skilled in a given field to overestimate their ability, while people who are more skilled in a given field tend to underestimate it. This is because less skilled people don't have the experience and comprehension necessary to accurately gauge their own ability or compare it to others, and very skilled people are aware, to a much broader degree, of the range of abilities they lack -- or assume that because it's easy for them, it must be easy for other people, too.
It's not a universal phenomenon -- largely the opposite seems to occur in East Asian subjects, which suggests that the phenomenon is a symptom of American, or more generally Western, culture. It also tends not to show up when people are estimating their capacity or odds in contexts that are perceived as much more difficult. This is called the worse-than-average effect, and applies to things like juggling, riding a unicycle, or living past 100.