AS mollymoo says, the rear fan usually blows air out.
What about the fan on the heat sink - where is that - usually, it will be at the front of the heat sink, blowing air through it, towards the exhaust fan, which will in turn be blowing that hot air out the back.
A fan in the front will blow air into the case, providing a source of cold air aimed, ideally, at the CPU. Many believe that you need an intake fan with a dust filter to blow in more than gets sucked out, thus creating a positive pressure, meaning that the excess air will be pushed out via any little holes in the case, around the optical drives etc, so helping keep dust out. In my old pc there was always dusty marks around the floppy drive and cd drive - even any case joints - all signs of a negative pressure, meaning dust was being sucked in by the exhaust fan.
