Friction has made fires, hindered motion, and inserted itself into traditional life in a multitude of ways. Molecular attraction between two surfaces. The molecules actually form a slight electromagnetic bond with each other. Rough surfaces interlock with each other. The rough spots on one surface mesh with the rough spots on the other.
Material is ripped from each surface, and there is a constant chattering between the two materials.
Static friction . Two surfaces are in contact, but are not moving against each other. Two surfaces are in contact and are moving against each other. Static friction is greater than kinetic friction. As the sled is moving, inertia is overcome.