Define frictional force and give four advantages, disadvantages and ways of reducing friction

A frictional force is one which opposes the movement of a body. eg air resistance, water resistance to movement.
Advantages : stopping cars in an emergency, slowing down skydivers to land safely, slowing down space craft on re-entry to the Earth’s atmosphere, enable us to walk!
Disadvantages: causes skin burns when you slide on any surface, limits the speed of cars when you want to accelerate, causes old hinges to get noisy, stops bicycle wheels from turning as fast as possible.
Best ways to reduce friction is to use an oily lubricant, like WD40 or similar.