The blowby vapors that end up in an engine's crankcase contain moisture as well as combustion byproducts and unburned fuel vapors.
The crankcase is sealed to prevent the escape of these gases into the atmosphere, but the vapors must be removed to prevent oil contamination that leads to sludge formation.
The positive crankcase ventilation (PCV) system siphons these vapors from the crankcase and routes them into the intake manifold so they can be reburned in the engine.
The main component in the PCV system is the PCV valve, which is usually located in the valve cover. A hose connects the PCV valve to the intake manifold. A second hose between the air cleaner and crankcase or other valve cover (V6 or V8 applications) provides fresh air to help flush the vapors out of the crankcase. Some engines have a separate air filter for the PCV breather hose located inside the air cleaner.
The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that's siphoned from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a leaning effect on the fuel mixture much the same as a vacuum leak. So air flow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blowby is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase.
It's important to note that PCV valves are sized for specific engine applications. The wrong PCV valve for an application can flow too much or too little air causing driveability problems. Varnish deposits can clog the valve, so replacement for preventative maintenance is recommended (every 50,000 miles usually).
Not all engines have PCV valves. Some (like Ford Escort, GM FWD cars with the Quad Four engine, etc.) ventilate the crankcase with a small breather hose and calibrated orifice. There is no spring-loaded PCV valve. On these applications, no maintenance is usually necessary.