A 4 piece bed set typically includes a fitted sheet, a flat sheet, and two pillowcases. This is a convenient way to buy bedding, as all the pieces coordinate and match.
When you purchase a bed set, it typically includes a bed frame, mattress, and bedding. The bed frame is typically a wooden frame with metal rails that the mattress sits on. The mattress is a thick, cushioned pad that provides support and comfort. The bedding includes sheets, a comforter, and pillows.