They are both basically oil changes. An A service includes a couple of quick safety inspections. It is only ~1 hour of labor.

The B service is still basically an oil change. It includes more extensive inspections, such as brake pads, serpentine belt, engine air filter, wheels & tires, etc. The dealer is supposed to inspect and replace if required. Some dealers will change the cabin filter or wiper blades at the B service. Prices seem to vary, as does the content of the service.