Through all the cracks around your doors, windows, floor boards, walls, ceilings, etc. It also enters and leaves through vents (generally in your bath rooms and kitchen). In the winter a lot of air comes in through your floor (if you have a crawl space) and out through your ceiling / attic. If your home is sealed well, you may not get much air exchange with the outside. However, most homes have quite a bit of air flow that the occupants aren't generally aware of.