Yarn can be made from any number of natural or synthetic fibers.
The most common plant fiber is cotton, which is typically spun into fine yarn for mechanical weaving or knitting into cloth. The most commonly used animal fiber is wool harvested from sheep. For hand knitting and hobby knitting, thick, wool yarns are frequently used.
Other animal fibers used include alpaca, angora, mohair, llama, cashmere, and silk. More rarely, yarn may be spun from camel, yak, possum, qiviut, cat, dog, wolf, rabbit, or buffalo hair, and even turkey or ostrich feathers. Natural fibers such as these have the advantage of being slightly elastic and very breathable, while trapping a great deal of air, making for a fairly warm fabric.
Other natural fibers that can be used for yarn include linen and cotton. These tend to be much less elastic, and retain less warmth than the animal-hair yarns, though they can be stronger in some cases. The finished product will also look rather different from the woollen yarns. Other plant fibers which can be spun include bamboo, hemp, corn, nettle, and soy fiber.