What is the difference between ac filters and furnace filters?

Actually, there is no real difference. So if you need an air conditioning filter, you can use a product that is sold as a "furnace filter." The important things to know to purchase them are the dimensions and the quality of filtration you need. However, additional filtration can only be provided to a point, and in fact, the better the quality of filter, the more often it will need changed. The more restricted the airflow, the more likely the equipment will suffer early failure.

The main purpose of the filters is to keep the big stuff out and help protect motors, coils and/or heat exchangers, not purify the air. The filters are there to protect the equipment, not people. The filters help keep out things like hair, big balls of lint, and foreign objects like parts of toys. Some companies may use deceptive marketing to suggest that their filters will purify the air.