what is the distributor o ring, and rear main seal

The distributor o ring is described here http://hondaswap.com/general-tech-maintenance/how-change-distributor-o-ring-81866/. The rear main seal is at the rear of the engine , where the crankshaft couples to the transmission. Incidentally a badly leaking distributor o ring can appear to be a leaking rear main seal since the distributor is located directly above the rear main seal.