Ring spun (combed) cotton is smoother and longer than open end yarn. The term “ring spun” means that the yarn goes through additional spinning processes designed to thoroughly soften and straighten each fiber. The result? Softer, more refined fibers (aka pieces you want to wear). Combed Cotton is softer and silkier to the touch, and it tends to be more expensive, because a higher volume of cotton is removed during processing.
The extremely soft, strong material is ideal for making bed linens and clothing which will be worn against the skin. If the textile used in a cotton product is combed cotton, it will usually be clearly identified. Many companies prefer to use combed cotton for clothing and bed linens because of the softness and tensile strength. Cotton which has not undergone the combing step tends to be more rough, and it will be more subject to fraying, pilling, and tearing.
(Excerpts from http://www.wisegeek.com/what-is-combed-cotton.htm and other Internet sources.)