Sheep are the principal source of natural animal fibre and there are more than 200 breeds of sheep worldwide. The largest number of breeds in one country is in Britain, with around 65. Read more about these on the sheep breeds page.
According to Wikipedia, wool is fibre derived from the fur of animals of the Caprinae family – sheep and goats, especially sheep, and for many it is only fibre from sheep. Unlike silk, it is formed of the protein ‘keratin’ and it has two qualities that distinguish it from hair or from fur: it has overlapping scales and it is crimped. Read more about wool here.