at low velocity, flow is smooth, with individual fluid particles flowing parallel to each other. At higher velocity flow becomes turbulent. In general the flow will still be in the same direction but there will be a small chaotic component. The interesting part is that the chaotic component (turbulence) often makes the flow more efficient.

To get really dorktastic... if you solve the Navier-Stokes equations (the equations that govern fluid dynamics) for a smooth geometry you will always get the laminar solution. You have to intentionally introduce a disturbance to get the turbulent solution. In terms of turbulence it is the small imperfections that really matter.