Slide means move over a surface while maintaining smooth continuous contact. It usually implies rapid easy movement without loss of contact with the surface. An example is the children playing with the slide in the park. When a child slides, she has a continuous contact with the slide until she reaches the bottom. A slide occurs when material remains fairly coherent and moves along a well-defined surface.
Flow is to move or run smoothly with unbroken continuity, as in the manner or characteristic of a fluid. This is can be further understood by considering the river as example wherein the water is flowing or running smoothly and continuously. A flow occurs when material moves down slope as a viscous fluid. Most flows are saturated with water and typically move as lobes or tongues.