To simply put it a newtonian fluid is a fluid that does not change its viscosity when force/pressure(shear stress) is applied on it at a constant temperature. A good example of such a fluid is water. A non-newtonian fluid on the other hand is the opposite of a newtonian fluid. Its viscosity is dependent on the pressure(shear stress) being applied at a constant temperature. It is important to note that temperature independently influences the viscosity of a fluid.