A normal, piston-driven, internal combustion engine, or long block, is composed of a short block and a head. The head is the top portion of the engine, containing the cams and valves, while the short block is the rest. The short block has two main chambers, a crankcase (where the crankshaft, rods and pistons are, all oiled up) and a combustion chamber (in which fuel and air combust). These two chambers are separated, essentially, by piston rings. When an engine revs, the crankshaft spins and moves the rods and pistons up and down. The resulting change in pressure in both the combustion chamber and the crankcase is the reason why a PCV (Positive Crankcase Ventilation) valve, and the PCV system in general, is needed.
When a rod and piston move up, the pressure in the combustion chamber increases (because the volume of the combustion chamber is decreasing). The increased pressure is the reason why fuel and air combust the way they do. This combustion drives the piston and rod back down; this action causes the crankshaft to spin, continuing the cycle. When a rod and it's piston are forced down from the force of the combustion, the pressure in the crankcase rises.
I mentioned earlier that the crankcase and the combustion chamber are separated by piston rings, or piston seals. These rings do not fully separate the two chambers until the engine is up to operating temperatures and the rings have sufficiently expanded (which is why it's always a good idea to let your car warm up before driving it). When the rings have not fully expanded, oil (from the crankcase) may enter the combustion chamber, and fuel and air from the combustion chamber may enter the crankcase because of the pressure changes that occur during a normal engine rotation. When the rotation of the crankshaft forces a rod and its piston upwards, the pressure in the crankcase drops, and if the piston rings have not fully expanded yet, a minuscule amount of fuel and air can enter the crankcase. With time, this fuel and air (called blow-by) may build up and ultimately damage the engine. Thus, the modern internal combustion engine contains a PCV valve which transports this blow-by back into the engine's intake, rather than keeping it in the crankcase (as is common on older cars) or venting straight to the atmosphere (as most race cars do).
The PCV Valve is only one part of the entire PCV system. This valve regulates when the system is in effect. The PCV system is not always in use. At idle and low rpm's the system is usually running, and the valve is open. This is because at low rpm's and idle, the pressure in the intake manifold is lower than the pressure of the crankcase, so the blow-by gases vent themselves naturally. At higher rpm's and at full throttle the PCV valve is usually completely closed (as there is nearly no vacuum), and the system is on hold. It's okay though, a breather tube performs the same operation as the PCV system at higher rpm's and full throttle.