sheets are a higher thread count- often cotton/poly blends with a (permenent press) chemical additive.
you can make your own sheets using any fabrics you want-
cotton/poly blends tend to be a little more (permenent press)
your regular cottons come out of the wash/dryer all wrapped up-frayed because of the raw edged- if you use pinking shears or stay stitch the edges the fray can be kept to a minimum.
i've purchased 116"wide batiks to make sheets for our bed- they make beautiful (designer) sheets at a much better price than i can find in the stores.