by 
The Curious Bag
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All modern engines use some sort of crankcase evacuation system. The PCV (positive crankcase ventilation) valve is the device that allows internal pressure to escape, without allowing the oil to follow.
Crankcase Pressure
All piston rings will leak a certain amount of pressure into the crankcase. If this pressure is allowed to build unchecked, it will disrupt oil pressure and cause gaskets to blow out.
Location
PCV valves can usually be found mounted on one of the engine's valve covers.
Engine Vacuum
As the pistons descend in the engine, they create a vacuum that draws in fresh air and fuel. As the pistons draw out of phase with each other, the most constant vacuum source in any engine is in the central portion of the intake manifold.
Harnessing Vacuum
The easiest way to prevent positive crankcase pressure is to subject it to a constant vacuum. The source for this vacuum is the engine intake manifold, which draws air through the PCV via a rubber hose.
Other Vacuum Sources
Depending on camshaft design and tune, some engines may not develop enough steady vacuum to evacuate the crankcase, and will utilize an auxiliary vacuum pump to do the job.
Source:
Resource:
Replacing a PCV Valve (Video)
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