Another good site:
Paragliding aerodynamics | Sci Fix
After reading your post I decided to read up on it myself. I liked the above site because it brought back most of an introductory aerodynamics lecture I've had before.
I'll make some feeble attempts at identifying some key points though.
*The boundary layer thickness describes the distance above the wing where the velocity of the airflow goes from zero (at the surface) to near 99% free-stream velocity.
*Free-stream velocity is the velocity of the fluid/air far away from the influences of the airfoil.
* Bernoulli's principle states that as a moving fluid meets a restriction the velocity must increase and to obey conservation of energy the pressure must decrease. For example I like to think of putting my thumb over a water hose. A certain mass flow rate has to exit the end of the hose because the pressure driving it out. When I put my thumb over the hose the same mass flow rate must go through this tiny restriction so it must accelerate. Since it must accelerate the pressure must also drop.
*For a wing, think of the wing as a restriction and the free-stream air above it as another restriction. A certain mass flow rate of air must flow through this restriction that is the wing and free-stream air above. Sometimes I like to think of a wing as half a venturi where the air above replaces the missing half.
*When the air first meets the wing leading edge it must accelerate. When it reaches it's maximum velocity the boundary layer is thinned and hence the smallest restriction to go through. This can sometimes be about 1/3 to 1/2 of the chord back from the leading edge.
*After this max velocity point above the wing the boundary layer will get thicker in general and the air must slow down because there is less of a restriction for the air.
*So this decrease in velocity means kinetic energy is reduced and potential is increased. We don't want this because we want to keep as much kinetic energy as close to the wing as possible.
*Eventually farther back on the wing the layer will become thick enough and the air low energy enough such that higher energy air outside the boundary layer will overcome and begin to push air backwards over the wing near the surface. This is where turbulation occurs which takes a lot of energy to do, hence drag.
*Also a larger turbulation area means less of the wing is able to do it's job (make lift).
*When there is more separation and turbulation the overall flow shape of air flow over the wing is much more different than what's required for adequate performance. It virtually changes the shape of the airfoil, so careful airfoil design, use of VGs, suction systems, and zig-zag tape are all efforts to control separation.
*When you see VGs and zig-zag tape it is in effort to take high energy air from the leading edge and transport it to the trailing edge with little vortices. This is to prevent the boundary layer air near the trailing edge from loosing too much energy and getting easily separated and turbulated. (I'm still slightly confused by this myself)
*These devices make drag themselves but the benefit is the boundary layer is made thinner than before and separation/turbulation is delayed until much further back toward the tailing edge which reduces drag a lot.