What is viscosity ? What is the cause of it ?

Explanation

In alaminar flow , any two consecutive layers of fluid have relative velocity between them.
As a result resistive force is produced tangentially at the surface of the layers in contact . This internal force of friction is known as viscous force . The property of fluid due to which relative motion between two consecutive layer is known as viscosity of the fluid.

A layer of liquid sandwiched between two parallel glass plates in which the lower plate is fixed and the upper one is moving to the right with velocity v
Suppose a fluid like oil enclosed between two glass plates as shown in figure .
The bottom plate is fixed while the top plate is moved with a constant velocity v relative to the fixed plate.
The fluid in contact with a surface has the same velocity as that of the surfaces . Hence , the layer of the liquid in contact with top surface moves with a velocity v and the layer of the liquid in contact with the fixed surface (stationery ) have velocity zero .
The velocities of different layer increases from bottom to top uniformly (zero velocity to velocity v).
For any layer of liquid , its upper layer pulls it forward while lower layer pulls it backward . This reslults in fore between the layers.
This type of flow is know as laminar.
When a fluid is flowing in a pipe or a tube , then velocity of the liquid layer along the axis of the tube is maximum and decrease gradually as we move towards the wall where it bocomes zero This is shown in figure.

Velocity distribution for viscous flow in a pipe
(Note : In a steady flow different layers side over each other without getting mixed up . This kind of flow is called laminar flow)