The internal resistance of the fluid that obstructs the flow of fluid is called viscosity. But all real fluids have some viscosity. To maintain the flow of a viscous fluid. We have to apply an external force against the viscous forces. The pressure difference between two points of a tube is necessary to maintain the fluid flow.
Consider a liquid, flowing at a steady rate on a horizontal surface. The liquid can be assumed to be composed of different horizontal layers. The velocity of liquid at any point on any given horizontal layer is the same. But the velocity of liquid in the layers perpendicular to the direction of flow is different in different layers. The velocity of the upper layer of the liquid surface is maximum and that of the lower layer is lesser. Similarly the velocity goes on decreasing as the depth increases and finally the deepest layer in contact with the horizontal surface is at rest. So, there exists a velocity gradient perpendicular to the plane of the fluid. Let us see the definition for viscosity in fluid mechanics.
If a specific layer of a liquid is taken, the layer below it moving with lesser velocity, tries to decrease the velocity of upper layer due to cohesive forces between the molecules of adjacent layers. In turn the upper layer which is moving with greater velocity tries to increase the velocity of the lower layer. Thus between parallel, successive layers of a liquid in motion, opposing force comes into play tending to decrease the relative velocity between the layers. The force is called viscous drag. It looks like a tangential shearing force is acting between the layers. To overcome these forces and for maintaining constant velocity between layers, an external force need to be applied. If there is no external force, the velocity of flow decreases and becomes zero.
This property of a fluid which opposes the relative motion between different layers is called viscosity. This is the internal resistance of friction exhibited between the layers of a fluid. Liquids such as kerosene, alcohol, water etc., which flow easily are said to be mobile, while liquids like tar, glycerin honey etc which do not flow readily are said to be viscous.