Sheep's hair or wool has been spun into fiber and woven or knit into woolen fabric since before humans began recording their history. Wool is a good insulator and holds heat well even when wet.
Some people find fabric woven from sheep's wool to be itchy. It can also be warm and soft.
Wool is sometimes blended with other materials to keep its warmth, but reduce the less desirable effects of wool.
Wool will shrink if washed or dried at too high a temperature.
Of non-synthetic fibers, wool probably holds heat best.
Newer treatments and techniques for making wool cloth can reduce or even eliminate some of the "traditional" disadvantages.
Wool has been worn by humans for over 10,000 years. It's one of the oldest fabrics, but is also used in some of our newest endeavors. Wool fabric is worn by astronauts in space.
- Wikipedia's article on wool
- International Wool Textile Organisation (I.W.T.O.) on the History of Wool