White STEAM from the exhaust indicates coolant entering the combustion chamber from somewhere [head gasket/cracked head etc.]

White SMOKE from the exhaust indicates possible ATF [Automatic Transmission Fluid] entering the combustion chamber [Transmission shift modulator valve diaghram leak]

Remember that under certain conditions white STEAM will come from the exhaust due to condensate forming in the exhaust pipes, this usually occurs during cool mornings during humid conditions like after a rain storm and will be most evident during the first few minutes of vehicle operation after a long period [overnight] of non operation, this is normal and will go away after the vehicle fully warms up after a good hard drive down the freeway.