"A nail file is a tool used to gently grind down and shape the edges of nails. They are often used in manicures and pedicures after the nail has been trimmed using appropriate nail clippers. Nail files may either be emery boards, ceramic, glass, crystal, plain metal files or metal files coated with corundum (sapphire)."
See more
@Wikipedia
"First of all remember to file only dry nails, because wet nails are more vulnerable and more easily damaged.
The file should move only in a single direction. An exception can be made for extremely brittle and weak nails, which can be saved only through cross filing."
Credit
More Info...