When two objects are brought into contact the molecules from the surface of one object get very close to the molecules on the surface of the other object. This results in forces of attraction between the molecules and this must be overcome so that one surface can move over the other.
Objects moving through fluids such as air or water also encounter frictional forces which reduce their motion. This is known as drag.
Friction prevents objects from moving or slows them down. It also causes wear on surfaces as they rub against each other and generates heat. Thus energy is wasted in overcoming friction.
Oil is used as a lubricant and provides a thin layer of liquid which separates the moving surfaces.
Ball bearings reduce frictions by making the moving surface roll rather than slide.
By designing the object to allow the easy flow of fluid around it (streamlining) as is done for rockets and racing cars the drag can be reduced considerably.
Uses of friction
Although in a lot of cases friction is a hindrance but it is also very essential. In order to walk we depend on the friction between the soles of our feet or shoes and the ground. The tyres and brakes on vehicles depend on friction to stop, slow down and start moving. It is the air resistance or drag that slows down a parachute.