A flange lock nut has a circular base flared out on the bottom of the nut.  The base acts as a washer but does not spin like a washer.  These lock nuts are used to cover a larger surface area or oversized holes.  A larger surface area of the nut allows for less pressure on the nut making it harder to loosen or damage.  Flange lock nuts are available in serrated and non-serrated.  Serrated lock nuts form to the connected material better which prevents loosening.  These work well with soft materials.