Knowledge and skills. It's difficult enough to distinguish these aspects of human achievement in English, but in Slovak there are three words: vedomosti, znalosti and zručnosti. Not everybody knows, three into two don't go, so we'd better focus on the English words. Knowledge is more mental and skills are more practical. In figure-skating, it's one thing to know (tell somebody) which edge to take off from and which foot to land on to complete a triple Salchow, but it's much more difficult to do one (show everybody).

It's similar with a foreign language: it's one thing to tell students the rules of grammar or the meaning of words (teaching), but more difficult to get them to use those meanings together with those rules (communication). Luckily it's not as difficult as figure skating though, or swerving between some hulking defenders to score a goal in ice-hockey. Imagine where they would be if they mixed up knowledge and skills.