Pins are a metal insert that goes into the back of some arrows to give the arrow a universal fitting nock. All pin nocks use the same size connector so it's easy to use the same nock on different arrows when they all use Pins.
Once you have chosen your Pins you can purchase Pin Nocks in this category
Nock Bushings fit into the back of some arrows to step down to a more standard nock size. In most cases a G nock (166 or 12 size) or a Super Nock Size (244 or 19 size)
You can buy Insert Nocks here