Split rims have a completely removable outer rim on the outside face of the wheel. It is designed to be pried off of the rest of the wheel after the tire is deflated. This way the tire is easily slid off or on to the main part of the wheel with out having to pry it over the rim. It is really handy for extremely stiff heavy duty tires.
Be VERY CAREFUL with split rims. The down side is that the split rim its self can explosively blow off of the wheel while the tire is being inflated if the split rim is not perfectly seated in its grove. People have been killed by these. There are other threads here discussing this aspect.
