​Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.
Friction depends on the nature of surfaces in contact.
For a given pair of surfaces friction depends upon the state of smoothness of those surfaces.
Friction depends on how hard the two surfaces press together.
Static friction comes into play when we try to move an object at rest.
Sliding friction comes with play when an object is sliding over another Sliding friction is smaller than static friction.
Friction is important for many of our activities.
Friction can be increased by making a surface rough

​The sole of the shoes and the tyres of the vehicle are treaded to increase friction.
The friction is sometimes undesirable.
Friction can be reduced by using lubricants.
When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than the sliding friction.
In many machines, friction is reduced by using ball bearings.
Fluid friction can be minimised by giving suitable shapes to bodies moving in fluids