What is the different between water jacket and radiator?

Water jackets are the hollowed out spaces in the engine block and head through which the coolant flows. In water jackets, waste heat generated by the engine is picked up by the coolant. After flowing through the water jackets, water flows through the radiator. The radiator is basically a heat exchanger which, through a system of tubes and fins, transfers this waste heat from the coolant to the atmosphere. After the water is cooled in the radiator, it flows back into the water jackets of the engine to pick up more waste heat.