What do these two terms mean?  Where did they come from (what language)?

They are German.  Zugzwang refers to the point at which every move causes (hopefully) your opponent's position to worsen.  Zwischenzug is a "between move," like a check on the opponent's King or an attack on the Queen they have to respond to before meeting a threat or advancing your position.

Okay, thanks.  I always get them mixed up.