Nails are made out of keratin. Keratin is a tough, fibrous protein that also makes up hair, horns, claws, hooves and feathers. Forms of keratin may also be a component of the silk produced by spiders and caterpillars.
The cells of the nail begin as epidermal cells below the root of the nail. As more and more cells grow, they are pushed forward and form layers of thin plates that eventually keratinize. These cells are dead, but they are also strong, flexible and permeable.
Fingernails grow much more quickly than toenails, and nails generally grow faster during the warm months.