Stupidity is really such a broad term. Different people have very different meaning and interpretations of it. This makes it a difficult topic to discuss sometimes. There are those who are ignorant. These people just haven't been exposed to certain things and haven't learned a lot. These can be children, or just people who don't spend time learning extra information. These people aren't necessarily stupid or idiotic; they simply haven't gained knowledge in certain areas. This may be due to lack of experience or simple lack of resources. If these people are relatively smart they can usually recognize that they have a lack of knowledge. Stupid people, in this category, are the people who don't realize they are lacking knowledge, and constantly claim to know things (or even just think they know things) that they do not. These stupid people are generally the most stupid and the most annoying.
But then there are another type of stupid people that are much more bearable (at least in my opinion). These are the people who just happened to be born with a lower intelligence. They can't help the fact that they are "stupid", that's just the way they are. But generally these people have other factors that make up for their lack of intelligence. This is because the universe seems to have a way to balance most people out. For example; Some people are ugly, but really smart and funny; some people are anti-social and lacking of a sense of humor, but really smart and attractive. There are many different type of people in the world, and all are needed for society to thrive. But back to the point. These type of stupid people are generally pretty fun. Generally they know they are stupid because of constant teasing by the moronic people of our world, but occasionally they can be oblivious.