Your factory automotive air filter is made from paper, while K&N air filters are made from oiled cotton fabric. Paper air filters are inferior because when they build up with dirt and debris, there is less filtration and air is entering at a slower rate. This will then make the engine less efficient through depriving it of fresh air. K&N filters provide constant air flow whether they are dirty or clean and are designed to increase air flow over paper filters. Replacement air filters also come in foam type, but they are only offered from aftermarket companies with their air intakes.
Paper filters use several layers which are needed to properly filter the dirt and debris. This design does not allow for the best filtration due to its thickness. K&N performance air filters have several layers that trap the dirt which then becomes part of the cotton media. K&N replacement air filters can handle more dirt per square inch than paper air filters as well. The cotton fibers of the media are crisscrossed rather than having holes like paper. When the holes build up, the filter must be tossed in the garbage, unlike K&N air filters which can be cleaned and reused.
Service intervals are also a major difference between these two automotive air filters. Paper can only last 10 or 15 thousand miles, while K&N filters will last 100,000 before needing to be serviced. If you think about it, you will go through at least 10 paper filters before your K&N car filter needs to be serviced. That is a lot of money down the drain that could be spent elsewhere.