Ignorant is not the same thing as stupid. A smart person can be ignorant about a lot of things; in fact, we all are. I'm ignorant about Ethiopian culture, about the contents of the Andromeda Galaxy, about the financial success of silver mines in Chile, about the rules of advanced calculus, about the number of hairs on your left ass cheek, and about the evolution of the polio virus, just to name a few things. These are things that I just don't know a whole lot about. The knowledge is out there, but I have been ignoring it in my pursuit of other things in life, so I am ignorant with respect to such information. However, if I took the time to study any one of these things, I'm sure I could understand it, so I'm not exactly stupid.

In short, ignorance always refers to one's lack of current knowledge about a particular subject, while stupidity refers to one's lack of mental ability to understand a subject.