Fasteners (or Fastenings):
This is generally the broader term and refers to any hardware device that mechanically joins or affixes two or more objects together.
Fasteners are often associated with creating non-permanent or detachable joints, meaning they can be removed or dismantled without damaging the components being joined.
Examples: Screws, bolts, nuts, washers, rivets, staples, clips.

Fixings:
This term tends to be more specific to devices used to attach or secure an object directly to a surface (like a wall, floor, or ceiling), often with the intention of being more permanent or load-bearing in that specific context.
Fixings often imply a connection to a larger, more static, or structural element.
Examples: Wall anchors (like sleeve anchors, wedge anchors, chemical anchors), concrete screws, nails (when used to secure something to a main structure), specialized frame fixings.