Plumbing is a cyclical system; water comes up, then goes back down and the air in the pipes has to move with it. Because of this, (and also to allow the escape of less-than-appealing odors) every plumbing system has a vent to the outside, sized to allow for enough air to come into the system when the draining water displaces it.
After I finished installing the drain lines for our sink I realized that it illustrates this point really well. There is a line of pipe below that the water flows down and another line above that makes a circle between the vent going outside and the drain going to the sewer. As water flows through the pipe on the bottom and displaces air, the pipe above allows air into the system to replace it.
Side note: If your pipes seem to drain slowly and you hear a gurgling sound, it might not be a clog; it might just be that the plumbing isn't properly vented. If that's the case, there's something called an, "air admittance valve" that can normally be installed without too much trouble. It's designed to respond to pressure in the pipes and to allow air in without allowing sewer gasses out.