A sealed bearing is packed with lubricant (usually grease of some sort) and has the plastic or hard rubber insert between the inner and outer race to seal it. The seal serves two purposes: keeping debris out and the lubricant in. Some reference material that I have read says that sealed bearings aren't capable os as many RPM's as open bearings. I can see th theory behind this due to the friction of the seal, viscosity of the grease and tendency for the seal to become hot and fail.

Shielded bearings have a metal piece rather than the plastic seal that is intended to keep contamination out. I've found that these are generally pre-lubed also but the lack of a good seal means that light lubricants can flow into or out of them. Most RC engines have a metal shield on the outside of the front bearing to minimize air leakage and to help keep debris out of the bearing.

Personally, I think the rear bearing on an RC engine should be an open type. That way it gets the benefit of the lubricant in the fuel. Typically you only see open bearings in things like engines and gearboxes where they receive good lubrication and are protected from debris and moisture. Yes, a sealed bearing might work but I would think that its life would actually be shorter. Maybe I'm wrong, who knows.

As long as you use good fuel with plenty of quality oil and don't allow the bearings to corrode, they should last a long, long time. Of course that also means no excessive RPM, no excessively lean mixture and no overheating.

This site has some good information: http://www.reliance.com/prodserv/motgen/b5021.htm