Bed sheets come in three main types: fitted, flat or in a set. The fitted sheet lies directly on the mattress, has four sewn corners creating a specific length pocket & is sewn with elastic to hold it in place on the bed. The flat or top sheet, generally goes on top of the fitted sheet and is (like the name says!) sewn flat with a hem. The set includes both the fitted and the flat sheet along with 4 pillowcases.