There's a reason why this thread went to two pages: people use words in different ways. And that, in turn, is not necessarily because they're ignorant or (shudder) stupid.
Edwardsfire and Swedish Meatball gave pretty good definitiions. Ignorance is a lack of knowledge and understanding, either in general, or on a particular topic. What PrincessK'sMom mentioned, the refusal to learn or become knowledgeable about something, is willfull ignorance. A good example is Sherlock Holmes, when he tells an astounded Dr Watson that he (Holmes) doesn't know and doesn't really care if the earth revolves around the sun, or the sun around the earth, because one way or the other this doesn't really affect his work.
And Bluestar, you've got a winner in your son. I wouldn't agree that racism is ignorance (although it is largely the result of ignorance), but I definitely like his way of thinking!