friction

1. friction, actually, is due to ahesive force between two different molecules, those of the sliding object and those of the surface.

When the object is at rest and a force is pressing the object onto a surface (because of the weight of the object or the like), molecules of the object and molecules of the contact surface are bound together by adesive force (similar to a "welding effect"). The strength of the adhesive force thus produces a resistance for the object to start motion. This is static friction.

When the object is in motion, adhesive force is still present. Because of the motion of the object, the ahesive force between molecules of the object and surface doesn't have sufficient time to buld up as compared with situation when the object is at rest. Thus the "welding effect" is somewhat reduced. The kinetic friction becomes smaller than the static friction.
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2. The coefficient of static friction is the ratio of the frictional force to the normal reaction of the object on the surface.