Define rolling friction. What is the cause of rolling friction?

Roiling friction is the friction which comes into play between two surfaces, when one body rolls over the surface of other the body. 
                 

(i) When a wheel or rolling object is placed on the surface, it depresses the surface and causes a slight elevation in the surrounding of depression as shown in the fig. above.
Therefore, body has to continuously climb up the hill and force is required to do so.
(ii) There are adhesion forces between rolling body and the surface. So, the rolling body has to detach itself from the surface on which it rolls and the force is required to detach the body.