These are important for RC use. The Closure is the shield at both ends of the bearing, protecting the balls inside from dirt, moisture and even impact.

Metal shields (named "ZZ")
Are shields of metal that does not have contact with the inner ring. Positive for RC is the low friction (none), but metal shields will not seal completely so small particles may enter the bearing. Recommended for on road cars.

Contact Shields (named "2RS" , "2" for both sides)
May be in a variety of plastic/rubber materials ,usually Nitrile Rubber, but will close the bearing completely, and thus protecting the bearing from moisture.
Recommended for off-road bashing.

Non Contact Shields (named "2RU", "2" for both sides)
Also made of various plastics/rubbers, usually Nitrile Rubber, but does not make contact with the inner ring. Nevertheless it has better protection than the metal sheilds, but not the friction that the Contact Sheilds have
Recommended for off-road racing where bearing wear is accepted.