Internally threaded  Metric DIN 934 Hexagon Nuts are the most common standard for metric hex nuts used in many applications where a metric nut is required

Internally threaded fasteners which have have a smooth rounded solid dome that covers the hex nut base. The surface of the dome has a low profile that protects the bolt threads underneath.

Internally threaded fasteners designed with slots or "castles" around the top, allowing for the insertion of a cotter pin or safety wire to prevent loosening due to vibration or rotational forces. 

Prevailing torque lock nuts feature a permanent undersized non-metallic insert (nylon/polyamide) that increases friction between threads, improving resistance to loosening.

Internally threaded fasteners denotes a type of full hexagon nut, meaning it's a standard, full-height hexagon nut, typically made of steel used particularly in woodworking application

Square nuts with either a standard or slim profile feature four sides, offering an enhanced gripping surface. They are typically paired with square head bolts to ensure better rotational stability.

Internally threaded elongate nuts whose height is 3x the major diameter of the thread. These are often used to join two externally threaded fasteners usually threaded rods

These have a circular washer like flange shaped base that increases the weight bearing surface area . Available with or without serrations (reduces loosening dur to vibrations). 

 AKA Stover Lock Nut, Top Lock Nut, Crimped Lock Nut, Oval Lock Nut. They have a conical top with chamfered corner.  The resistance to loosening is created by distortion in the top threads