White smoke coming from a car's tailpipe means that somewhere moisture is being burned off. However, it can also indicate a more serious issue if the coolant or gas is the moisture being burned off. If the smoke disappears shortly after starting, there is likely no problem, but if it continues, it means that water or coolant are in the combustion chamber.
The most common serious cause of white smoke coming from a car's tail pipe is a leaking or blown head gasket. As a person is less likely to notice the color of smoke coming from their car's tail pipe, another common symptom of this problem is overheating. If the car is overheating regularly and emitting white smoke, the culprit is almost certainly a blown head gasket.
The head gasket works as a seal between the engine block and cylinder head. When this seal is broken, water or coolant is able to enter the combustion chamber. The water turns to steam in the chamber and comes out of the tailpipe as white smoke.
A more dangerous reason is fuel in the intake from a faulty fuel delivery system. This also exits the tail pipe in the form of white smoke and smells like gas. The danger is that it could ignite in the hot muffler.