Friction:
When a body moves or tends to move over the other body, a force opposing the motion develops at the contact surfaces. This force that opposes the movement or the tendency of movement is called as frictional force or simply friction. Frictional force acts parallel to the surface of contact always, opposite to the moving direction and is dependent on the roughness of surface.
A frictional force develops when there is relative motion between body and surface on application of some external force.
A frictional force is dependent upon the coefficient of friction between the surface and the body which can be minimized up to very low value equal to zero (theoretically) by proper polishing the surface.