As everybody knows, cotton is a natural fiber extracted from the plant of the cotton. If you manufacture this natural fiber by spinning, you will obtain cotton yarn that can be transformed in the textile you want. The fiber is more often spun into yarn or thread and used to make a soft breathable textile.
In underwear items we prefer lighter fiber, manufactured in Jersey, or ribbed embellished with a little percentage of Polyamide which will give elasticity because cotton is a 100% stiff fabric.