fitted sheet vs bed sheet

fitted sheet and bed sheet both are nouns.

Word	Noun	Adjective	Verb	Adverb
fitted sheet	Yes	No	No	No
bed sheet	Yes	No	No	No

As nouns, bed sheet is a hypernym of fitted sheet; that is, bed sheet is a word with a broader meaning than fitted sheet:

fitted sheet: a sheet (usually with elastic edges) tailored to fit a particular mattress
bed sheet: bed linen consisting of a large rectangular piece of cotton or linen cloth; used in pairs

Other hypernyms of fitted sheet include sheet.

fitted sheet (noun)	bed sheet (noun)
a sheet (usually with elastic edges) tailored to fit a particular mattress	bed linen consisting of a large rectangular piece of cotton or linen cloth; used in pairs