You may recall as a child walking in a nice straight line as a group of grade-schoolers. As you got older, you tended to travel a bit faster, and it was harder to keep the group orderly. Now, once class is finished, you all run pell-mell from the room in an almost random mob.
As we will discuss in the next lecture fluid flow involves transfer of momentum, whose resistance arises from friction within the fluid and with the surroundings (walls). If the resistance is sufficiently small, the fluid momentum is unbalanced and it "loses control" of itself, much like my students after class. In reality, both "mobs" stop being orderly because at high values of momentum any small fluctuation from orderly becomes magnified and difficult to bring back under control (since the resistance is too small to damp it out).
This leads us to the fact that there is generally thought to be three (really four) "regimes" of fluid flow:
Laminar flow is a "well-ordered" flow where adjacent fluid layers slide smoothly over one another (past one another) and interaction (material mixing) between these layers or lamina of fluid occurs only at the molecular level (i.e. as viscous stresses). Occurs at "lower" flowrates.
Turbulent flow is a flow characterized by random motion of fluid elements where each fluid element's velocity has a fluctuating nature. Occurs at "higher" flowrates and is often exploited for better mixing.
Transitional flow is a flow which exhibits both laminar and turbulent flow characteristics. Occurs at "intermediate" flowrates where the flow is transitioning from a purely laminar to purely turbulent regime.
Above, I mentioned that there is really four regimes of fluid flow because there is a flow that might be thought of as "super-laminar", often called Stoke's flow. This type of flow occurs when the damping force of "friction" (viscous effects) is so large that the inertia of the fluid is negligible. We will come back to this later.
Define laminar, turbulent, and transition flow regimes.