There are several causes of white smoke coming out the exhaust and it depends on the symptoms and the conditions of the car when the white smoke happens. The white smoke can either normal or simply means a problem of the engine.
Car Problem Symptoms
When the white smoke is seen just before the engine warmed up
This is normal. This happens because of the condensation that is formed on the engine when the engine is parked overnight. When the cold engine is started the condensation will steam off then comes out as white smoke from the exhaust of the car.
To prevent white smoke that comes out the exhaust before the engine warmed up, park the car inside a covered parking especially when the temperature is cold or when it is about the rain.
When the white smoke is seen at any engine temperature
This means that the head gasket is blown. A blown head gasket is one of the causes of white smoke, the white smoke is noticeable at any engine temperature such as when the engine have cold, warm or hot engine temperature. Head gasket is the one that keeps the water that is circulating the engine block to cool down the engine, from entering the engine cylinder. If the head gasket is damage, for example the gasket has crack or ruptured, the water will penetrate inside the cylinder this water will be burn as steam and the steam that is produced will comes out the exhaust as white smoke.
To fix the problem the head gasket must be replaced with a new one.