Maybe you should Google it, but here is the way I understand it:

The thing that causes air resistance is a pressure differential. The air pressure at the front of a vehicle is higher than at the back. Why? The air at the front moves smoothly and exerts a given pressure. The air at the back is turbulent, becomes "detatched" from the surface, and exerts less pressure on the back of the vehicle. That's why so much attention is spent on streamlining and making things "aerodynamic". If you could keep the air from becoming turbulent as it moves around a vehicle, the air would exert virtually the same pressure on the front and back, and there would be no air resistance. By the way, most cars on the road today (wedge in front and blunt in back) would be more aerodynamic if they were driven backwards!

When riding behind a bus (I hate busses) you are riding in a mass of air that is being somewhat dragged along by the bus.