If the flow is slow, you see a smooth clear stream. This is laminar flow, and if you were to put a drop of dye in the water at the top you would see it flow down in a straight streak. But if you turn up the flow, you will find the water is no longer clear, but roils and behaves like white water in a river. At this point, turbulence has set in, and the flow contains vortices and swirls that are difficult to describe.
If you have a slow flow of air, like a gentle breeze that moves a flag only slightly, you’ll get a movement like the laminar flow of water, a slow billowing motion rather than a flutter. If the speed of the air accelerates enough, turbulence sets in and the flag begins flapping.
It isn't a bad article, but I wish it would not start with a statement like this:
Would you believe that nobody really knows?
This is the same as lifting Feynman quote about quantum mechanics and how no one understands it. It gives a very false impression that we don't know anything at all about this phenomenon, which is patently false by the fact that the article did offer SOME form of understanding of the fluid dynamics that is involved. What should have been said is that we know something about it, but using the standard of physics, we need a full, solvable mathematical form to be able to say that we understanding it well enough. This is more accurate than claiming that nobody really knows why. We need to be a bit more careful in the words that we choose when conveying something to the impressionable public who only have a superficial understanding of not only science, but how science is done.