Radiators, water pumps, thermostats, hoses and heater cores.
The same system that keeps the engine from overheating in August is the one that puts heat in the cabin in January, so a problem shows up in one season or the other.
Overheating is worth stopping for immediately. Continuing to drive a hot engine can turn a hose into a head gasket very quickly.
Sweet smell inside the car, fogging windows, or coolant on the passenger floor usually means the heater core.