The crankcase in a car is used as a storage place for oil, usually in a pan located below the crankshaft. While the crankshaft and the oil aren't intended to come into contact (because if they did the oil would get frothed up like a thick, black milkshake), oil vapors can still find their way into the blow-by gases. It's not a good idea for these oil vapors to be recirculated back into the cylinders along with the blow-by gases because they make the gas-air mixture too combustible, equivalent to lowering the octane of the gasoline, which in some engines can degrade performance slightly and in older engines can even cause backfire when the gas-air mixture combusts prematurely. The oil vapors can also coat the air intake with an oily film, gradually clogging the air flow over time. If you don't drive a high performance vehicle, these problems aren't exactly crucial to your car's operation and the oil build-up can be scrubbed out periodically during maintenance, but some people (and some car manufacturers) prefer to have something that will scrub the oil out of the blow-by gases before they're recirculated in the first place. Enter the oil and air separator.
The idea of an oil and air separator is to extract the oil from the air before it's sent back to the intake manifold and put it someplace where it won't cause a problem, either back in the crankcase or in a small receptacle called a catch can. Not all cars come with built-in oil separators and not all cars necessarily need them, but they can be purchased as aftermarket items. And if you have the necessary DIY skills, you can even make one yourself. There are actually a number of different ways in which these oil and air separators can work. Probably the most common kind blows the oily air through a mesh filter. The oil droplets are trapped in the mesh while the air passes through. The most effective such filters are made up of microfibers, which can trap very small particles of oil. Alternatively, the air and oil filter may require the recycled gases to go down a tube with holes in its side. The lighter air molecules escape through the holes, while the heavier oil droplets fall all the way to the bottom, where they can be removed. And some advanced systems use a centrifuge to drive the heavier oil droplets out of the air. The oil coalesces on the sides of the centrifuge and can be channeled back into the crankcase.