Friction is the force that is created when two surfaces move, or try to move, across each other.
The amount of friction produced during this process depends on the texture of both surfaces and the amount of contact force (motion) pushing the two surfaces together.
Useful effects
Friction is the reason we are not slipping and sliding all of the place! Imagine walking across an ice rink... Without friction, this is what it would be like to walk down the street. Friction allows us to keep our balance.
It is the essential force at work during braking, and also produces heat, which is required in the action of lighting a match.
Harmful effects
Materials will wear down quickly if they are constantly exposed to the effects of friction. Think of the material on the bottom of your socks or the inside of your jeans, it tends to get far more worn than on the rest of your clothes.
The heat generated can sometimes be unwanted, and even unsafe. Fires can start if flammable materials become hot from the effects of friction.
