Covers: generic term for anything on a bed. Blanket: a piece of relatively thick cloth that helps keep you warm. Traditional materials for a blanket are wool, cotton, etc. Sheet: A thin piece of cloth that that is often fitted onto the bed. So, you lie on the sheet. There is often a 'top sheet' as well in the US, so you sleep on top of one sheet, and under another one. If the weather is hot, you might sleep just under a sheet, without a blanket. Traditionally, sheets are made of cotton. Quilt: Same function as a blanket, but they are often thicker. Also, they are made (the outside at least) of many, possibly random pieces of cloth sewed together.

In American English, "covers" refer to all bedding that are on top of you as you sleep. And blankets, sheets and quilts do not mean the same thing. Do an IMAGE SEARCH on the internet and you will see the words are not referring to the same thing. Hope that helps.

They are all bed-wear, but each one is different. You usually lie on a sheet, whereas the other three items go on top of/around you.