What is rolling friction?

While discussing friction, often after describing static and sliding friction, a third type called rolling friction is mentioned. The static friction acting on a rolling body can be either in the direction of motion or opposite to it depending upon the forces acting on the body. But the direction of rolling friction is generally not mentioned. By analyzing the process of rolling in the absence of any push or pull we come to the conclusion that the energy loss in rolling motion is due to the continuous collision of the rolling body with the body on whose surface it rolls.