what is the Newtonian, and Non-Newtonian flow ?

In a Newtonian fluid the shear stress is proportional to the velocity gradient (Tau = mu * du/dx, where mu is the viscosity). Most normal fluids are Newtoninan (air, water, ...)

In non-Newtonian flows this linear relation between stress and strain is not valid - for example, many paints become more "fluent" (less viscosity) under shear (when you paint) and then "stiffens" when the shear dissapears. Many polymer flows are also non-Newtonian, blood flows also. In non-Newtonian flows there is a non-linear relation between the stress and the strain (strain changes the viscosity)

Non-Newtoninan flows are often difficult to simulate.

Jonas, I have a suggestion. We are to encourage beginners and also ensure that the right message goes. So, why not we suggest text books for such beginners in place of giving kind of brief answers (because of limited space availability, etc.). They will be really enlightened I do believe.

Sure, you're free to suggest a good book instead.

Boundary layer theory by H Schlichting and Transport phenomena by Bird, Stewart and Lightfoot