static and kinetic friction

It is due to the electrostatic force between the contact surface.That is many tiny sharp point on the contact surface which will cause sharp point effect and accumulate charges.Hence,the charges attract the other charges on the other surface.When the object is moving, the electrostatic force between the contact surface will decrease since the symmetry is destoryed.

You may think in this way:
When a surface is moving over another surface, the contact between these 2 surfaces is not as tight as that when there is no relative movement. As the contact is less tight, the friction is less. So the kinetic friction is less than the static friction.