Any time there is relative motion between a solid and a fluid, a small region near the surface will see a large change in velocity. This region, shown with smoke in the image above, is called the boundary layer. Here air flows from right to left over a spinning spheroid. At first, the boundary layer is laminar, its flow smooth and orderly. But tiny disturbances get into the boundary layer and one of them begins to grow. This disturbance ultimately causes the evenly spaced vortices we see wrapping around the mid-section of the model. These vortices themselves become unstable a short distance later, growing wavy before breaking down into complete turbulence. (Photo credit: Y. Kohama)
When a fluid flows next to a solid, the fluid right next to the solid is always stuck to its surface. This means it has zero velocity with respect to the solid. This is called the no-slip boundary condition, and it’s why boundary layers exist.
The picture above is a special situation meant to make a beautiful and illustrative image. However, boundary layers exist everywhere: the wind blowing across your face, the water right next to you when you’re swimming, the air right next to your car when it’s driving. Since air is hard to see, in the picture they use smoke so you can see it.
As boundary layers continue across objects, they always go through a transition from well-behaved and layer-like to disordered and gnarly-looking. The word for layer-like is “laminar.” The word for disordered and gnarly is “turbulent.” In the picture you can see the laminar-to-turbulent transition very well.