Well I'll have to look it up now, but I see it as a substance produced by the body that isn't made of cells itself. Like any of the other excretions and things the body makes.

Edit: apparently both your nails and hair are mostly made of keratin, but keratin isn't produced and excreted to produce the nail and hair structures like a playdough factory like I imagined. Special cells are produced that are primarily keratin and they are added to like a chain and die/harden as they are pushed out from the body.