The class of fibers that come from natural sources, i.e., are non-synthetic.

These natural fibers are used in the textile industry to make yarn and/or fabric:

• Cotton: Fiber from the extremely hairy seed pods of the cotton plant, Gossypium hirsutum. Cotton is spun into thread which is used to weave fabric. It is also spun into yarn, and used to make twine. It is also used to make batting, which is loose fluffy stuff used to make quilts poofy. Cotton is very absorbent. It is also probably the most popular fiber in the world.
• Wool: The hair of mammals, usually sheep. Wool is usually spun into yarn (fatter than thread) which is knitted into sweaters, etc., but can also be used to weave blankets, fabrics for making coats and other warm clothing. Wool retains warmth even when it is wet.
• Linen: A bast fiber from the stems of the flax plant, Linum usitatissimum. Linen is almost never spun into yarn. It almost always appears in woven fabrics, and is associated with summertime attire because it breathes well.
• Silk: Fiber of insect origin, spun by the larval silkworm in the production of its cocoon. Silk is one of the most prized fibers. It's soft and drapes well, and feels great against the skin. Silk fabrics have a reputation for being delicate, but actually, silk is the strongest natural fiber. It is absorbent, warm in the winter and cool in the summer.
• Hemp: A bast fiber from the stems of Cannabis sativa
• Ramie: Bast fiber from the stems of the China Grass plant (Boehmeria nivea)
• Jute: Bast fiber from the jute plant, Corchorus olitrius. Jute fiber is rough, and used more for twine and carpet backing, rather than for garment textiles.

I used to be militantly pro-natural-fiber and would wear nothing else, until polartec was invented.