It's not always possible to tell the difference between types of sheets just by touch. Here's a glossary to help sort out the distinctions.
There are four main varieties of cotton staple, or fiber, the longest being the finest:
American Upland is the most common; a short- to long-staple cotton, it makes up 90 percent of the world's crop.
Egyptian cotton is a fine, lustrous, extra-long staple that is usually brownish in color.
Sea Island is considered the finest of all cottons; it's a silky, white, extra-long staple grown exclusively in the West Indies.
Pima, a fine but strong extra-long staple that is brownish in color, resembles a combination of sea island and Egyptian cottons.
These staples can be woven into various types of fabric:
Flannel originated in Wales, where it was called gwlamen ("allied with wool"). It is a loosely woven, heavy cotton noted for its softness, with a napped (raised) finish and considerable variation in weight and texture.
Muslin named for Mosul, a textile center in ancient Mesopotamia (modern Iraq), is now a generic term for a simple-weave fabric ranging from sheer to heavy sheetings. Fine muslin is smooth, with a 180- to 200-thread count.
Oxford, first produced in Scotland in the nineteenth century, is soft, porous, and rather heavy; it launders well.
Percale is named for pargalal, a centuries-old cloth from Persia; it is a finely combed, closely woven fabric noted for its fine texture and finish.
Sateen is made in a satin weave, in which warp (lengthwise) threads interlace with filling threads, resulting in a lustrous, smooth-faced, durable fabric.