You can't be a heretic - material or otherwise - if you don't know any better.
You have to have learned an article of faith, knowingly rejected it and persisted in the error in the face of repeated correction to be a heretic.
No, that's not true. You don't have to "know better" to be a heretic.
Material heresy is when you believe something contrary to the Catholic faith and did not know that it's contrary to the Catholic faith. The idea is no less dangerous, but the person is not guilty of heresy proper (i.e., the person is not a heretic) if he does not know that what he believes is contrary to the faith.
On the other hand, a formal heretic is a person who knows that Catholic teaching on X is Y, but decides to believe Z anyways.
To say that you have to 'know better' [to be a formal heretic] isn't the best way to phrase it. But to be guilty of heresy (to be a heretic, not merely hold an heretical view) you must be aware of the doctrine that you are contradicting.
It's troubling that anyone who has said this in this thread has been thumbed down. This is Catholic teaching.