WrapperThe word wrapper refers to a cover that surrounds something to protect and contain it. It is common to use foil, plastic, or some other unnatural material as a wrapper. In this case, it is a synonym for covering.
Ex: You've dropped the lollipop wrapper on the ground. Littering is nothing to sneeze at, so pick it up please.
Ex: My mother always packs my sandwich in a foil wrapper for school. I love and appreciate the effort she puts into keeping my food fresh for lunch.
The word wrapper can also refer to the jacket of a book. This is primarily a British word; the words cover or jacket are usually used in American English.
Ex: I would take the book's wrapper off before reading it. You will pay a high price if you return it to the library ripped.
Ex: I loved the artwork on the jacket of that book! The bright colours really expressed the wrath of the main character and served as a great preface to the novel.
CrustThe word crust refers to any hard, but breakable covering on top of something. It can be formed by environmental factors or to form a protective layer.
Ex: Scabs form as a protective crust over wounds. This enables them to heal without being touched by airborne bacteria.
Ex: There is an icy crust over the snow that fell in our yard. It is keeping the soft, powdery flakes from melting.
The word crust can also refer to the hard, but breakable outer layer of bread. Pies also have a crust to protect and contain their filling.
Ex: I have baked a fresh loaf of bread, but the crust is too hard and crumbly. I hope my effort was worthwhile and it tastes better than it looks.
Ex: Try not to break the crust of the apple pie! The warm filling will flow out and cool off before we have the chance to serve it.