Nuts have internal machine threads, for use with machine thread bolts and screws.
A standard six sided nut.
A low carbon steel for general use. Chrome plated and polished for a mirror like finish.
A hex nut with a nylon ring to resist loosening.
A high crown type of cap nut used for appearance. Bolts must be in the proper length.
For adding a machine thread to material that otherwise cannot be tapped.