According to HowStuffWorks, keratin is found in the outer layer of the skin, in the hair and in the nails. Keratin consists of strong protein fibers that protect the body from harm.
Keratin is tough and does not dissolve when exposed to most solvents. The keratin in the outer skin layer gives skin strength. Keratin in hair provides extra protection, and keratin in nails creates a primitive weapon and also increases dexterity in handling small objects. In animals, keratin also forms the horns and the baleen plates through which some whales strain out their food. Keratin fibers give insects' silk and spider webs strength. Tough keratin silk protects many insects from harm during their defenseless pupa phases.