Loose bearings are just that -- loosely installed in the hub, hopefully wallowing in good grease.
Sealed bearings have grease seals to keep the guts of the bearing separate from the outside world.
USUALLY, not always, sealed bearings are a part of a higher-quality hub/headset/BB/wherever.
Loose-bearing hubs are more easily user-rebuilt; pull the bearing balls out, clean them, pack in new grease, slip the balls back into the grease, and reassemble. Sealed bearings CAN be rebuilt, but are usually are just replaced.
I had a long post I had wrote and then deleted it when I remembered that Jobst Brandt had a great article on the hub types. So instead of some rambling post that will instigate a debate, I just give you the web site and you have to read it. "Sealed" Bearings by Jobst Brandt
I have several cup and cone hubs, one set is a Suntour Superbe with a grease port, these hubs have over 150,000 mile since 1983 when I bought them and the bearings have never been replaced. I simply insert my grease injector onto the zertz fitting and pull the trigger on the gun slowly, the grease then oozes out of the sides of the hub, and when the grease runs clean I'm done except for cleaning the grease off of course. I did this every other month when I was racing and semi annually to these hubs when I stopped racing since they were new. Campy Record hubs have the grease fitting, the only hub that I know of that does now.
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