They're the part on the front axle that go between the ball joints and the spindle or bearing hub. Here's one of my old knuckles attached to the axle:
The part on the axle tube that connects to the knuckle is called a yoke, or knuckle yoke.
That knuckle (and all knuckles used on sub 1-ton domestic 4x4s since the early 70's) is an open knuckle. That basically means that you can see the u-joint from the outside. Closed-knuckle fronts are enclosed, and are common in older axles or HD ones. AFAIK, Japanese fronts that use Birfield or other types of CV joints are closed-knuckle, since those joints have to swim in grease.
Generally, the closed-knuckle fronts used on HD axles like the D70 have a limited turning radius, which makes them less desirable.
my dana 60 is open but uses bearings instead of ball joints. the early closed dana 60 has coarse-spline axleshafts, which are weaker. most dana 70 fronts use the closed design. they also have a weak closed design and have only 23 spline shafts. also it is easier to see if a u-joint is about to puke. nothing to take apart if you want to see the joint. most closed use lower axle spline counts with smaller shafts. i choose opens.
westcoast rockhuggers offroad club