The
Roman Catholic Church teaches the
doctrine of
transubstantiation: the bread and wine
literally become the blood and body of
Christ during the giving of the
sacrament. (n.b. I know some
devout Catholics who
don't believe this. They aren't exactly
devout then, are they?)
Luther (and I presume Lutherans) however held a subtly different belief called consubstantiation. He believed that although the Real Presence of Christ is present in the bread and wine, it is only there in essence. I believe Luther likened it to the heat within an iron bar.
I knew studying the Reformation would be useful sometime :)