“Material vs. Formal:
Material means that something is there i.e. It exists. Formal means it is declared as such. If I say John Paul II is a material heretic, that means, he has all the matter of a heretic. It can be proven, that many of his writings, or things that he said, are heretical. I would not dare to say he is a heretic (formal) for no one can judge the Pope and no one can judge his intentions or conscience. I see heresy, it is there (material), but one cannot say it is formal heresy. For instance, the Pope will say “according to tradition…” and then he proceeds to say something wrong. An example of formal heresy would be if he should say something like “Contrary to what the Council of Trent says, I tell you…” some heresy.”
http://www.catholicapologetics.info/modernproblems/vatican2/documents.htm
There is also public vs private. I am trying to get a better handle on this because it absolutely affects the issue of the pope.