If you've been shopping for Egyptian cotton sheets, you've probably heard that the fiber length has something to do with the level of quality that you can expect in the end product. This has very much truth to it and it is in fact the single greatest factor in the determination of the quality of a cotton product.
The length of the cotton fibers is called the “staple”. This is generally known to be between 18mms in low quality cottons and upwards of 50mms in high quality cottons, such as those from Egypt and the southwestern United States. Information on other types of cotton can be found at Luxury of the Pharaohs.
Short staple cotton, which is that cotton which doesn't reach the fiber length of 18mms, is the lowest quality cotton available and it has a rough texture.
Intermediate varieties of cotton, between 18mms and 28mms, make up the bulk of the cotton fabric produced in the world.
Extra long staple cotton isn't that common, but it is produced in only a few places in the world. The industry standard for a cotton to be called “ELS” is 34.925mms or 1-3/8''.
The longer the staple, the less threads have to be woven together to give the yarn a high amount of strength. The sheets will last longer, they won't tear as easily, and they won't pill as easily as their shorter staple counterparts.