needle bearings are in many cases - uncaged and a typical application is in transmissons of cars -surrounding a shaft and inside a multiple gear (cluster)
roller bearings are typically - in a cage of some type .
typically, something holds the rollers apart.
Needle bearings in a retainer are the type of bearings used on model engine con rods .
this is a shell with the ends rolled around the ends of the needles and the exposed bearing surface is the inner portion .
hows that for useless info -