What is viscosity? What is the cause of it?

(N/A) Viscosity is the property of a fluid by virtue of which it opposes the relative motion between its adjacent layers.
Cause of Viscosity:
In a laminar flow,any two consecutive layers of a fluid have a relative velocity between them. Due to this relative motion,a resistive force is produced tangentially at the surface of the layers in contact. This internal frictional force is known as the viscous force.
Consider a fluid (like oil) enclosed between two parallel glass plates. The bottom plate is fixed,while the top plate is moved with a constant velocity $v$ relative to the fixed plate.
The fluid layer in contact with a surface has the same velocity as that surface. Thus,the layer of liquid in contact with the top plate moves with velocity $v$,and the layer in contact with the bottom (stationary) plate has zero velocity.
The velocities of different layers increase uniformly from the bottom to the top (from $0$ to $v$).
For any layer of liquid,the upper layer pulls it forward,while the lower layer pulls it backward. This interaction results in a resistive force between the layers,which is the cause of viscosity.