Not sure if this is a great graphic, but you can do a google image search for aerodynamics and probably find some more.
My Corvette was notoriously bad for this. The back of the car is completely flat which creates a huge low pressure area. It was good for aerodynamics (the car never needed a spoiler because of it) but every time I washed the car the back of the car would be FILTHY after driving it for 5 minutes because all the dirt and dust would get sucked to the back bumper like a vacuum.