Rolling friction:

Rolling friction is created when one object rolls over a flat surface and resistance is created causing force to be exerted on the object that is in motion. Rolling friction occurs when a ball, wheel, or cylinder rolls freely over a surface, as in ball and roller bearings. The frictional force, f, the force required to slow down the wheel produces the torque which tends to decrease the angular velocity, w. But, the surface could not possibly have such an effect on the wheel once the wheel has achieved pure rolling motion and constant angular and linear velocity.  Zero friction occurs for horizontal motion at constant velocity only, but it is non-zero for any case in which acceleration is occurring parallel to the direction of motion of the center of mass, as when the object is rolling-without-slipping up or down a sloped surface.  If we consider the rotation as being about the center of mass of the object, then the frictional force should be in the direction to provide the torque necessary to decrease or increase the angular velocity, depending on whether the object is accelerating or decelerating, respectively. The friction can be in the direction of motion or opposite to it. In pure rolling motion there is no slipping and sliding, thus the contact points have no relative motion. This results in a frictional force of zero.  Therefore, the wheel will roll forward with constant velocity,

v = Rw,

here R is the radius of the wheel.

The type of friction which is produced between the 2 bodies when the surface of 1 body moves by sliding against the surface of other body. It is the common observation that a body with wheels faces less friction as compared with the bodies of same size but which are without wheels. When a wheel moves on the smooth surface, it has to connect with the surface at a single point only. Thus; the sliding friction is almost zero. Wheel is compressed a bit temporarily at the connect point of the 2 surface under stress. This little sliding friction is called as rolling friction. The rolling friction is quite less than the sliding friction. It is nearly a hundred times less than that of the sliding friction. Thus, at the points of machines which have to slide against each other, ball bearings are used, which converts sliding friction into rolling friction.