What is the difference between nappe and thrust sheet?
Nappes and thrust sheets are very similar terms.

I agree with definition found on wikipedia:
(except for the 2 km requirement of movement, I would replace 2 km with significant distance.  I am sure you still have a Nappe with only 1.5 km of movement is measured.)  

In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi) or 5 km (3.1 mi) above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced (or "thrust") over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippe.