The valve train is made up of valves, rocker arms, pushrods, lifters, and the cam shaft. The valve train’s only duty is likened to that of a traffic cop. It allows air and fuel in and out of the engine at the right time. The timing is regulated by the camshaft which is synchronized to the crankshaft by a chain or belt
If your vehicle only sends out grayish/white smoke at start-up, it is possible you have a leaking valve seals. Valve seals prevent oil from above the valve from leaking into the combustion chamber. However this wear out over time, they can allow oil to seep into the combustion chamber and settle there until your start the engine again. Your oil does not leak past the valve seals while the engine is running since the seals expand with the heat of the engine and plug the leak.
A second common problem is the timing chain or belt that could slip or even break causing the camshaft to stop rotation. Bear in mind that the camshaft tells the valves when to open and if it stops spinning then the valves stop opening and closing. Thus no valve moving, no engine running.
http://www.automobileglobe.com/common-valvetrain-problem/