Figure (a) shows inviscid flow (the fluid is ideal) along a flat plate. Under this condition, the fluid simply slips over the surface with velocity V. No drag results if the fluid is frictionless (inviscid). Figure (b) shows viscous flow along a flat plate found under conditions of real fluid flow. A very thin film of fluid adheres to the surface. At the surface of a body, point B, the flow velocity is zero. As one moves away from the body, the velocity of the fluid gradually increases until at some point A the velocity becomes a constant value V. The layer of fluid where the velocity is changing from zero to a constant value is known as the boundary layer. Figure (c) compares laminar and turbulent flows. As is usual for turbulent flow, there is a random motion in the boundary layer.

Credits - NASA