One major category of forces in physics are contact forces, which includes friction, normal forces, applied forces and tension forces. The other category is forces that act at a distance, which include gravitational forces, electrical forces and magnetic forces.
Contact forces result from two objects coming into contact with one another. In the case of friction, this force occurs when one item drags or rubs against another item. Examples of friction include brake pads on a vehicle and air resistance. A normal force is an upward force exerted on an object that is equal to the weight of the object and helps balance the object. For example, if a person is standing the floor, the floor exerts a normal force upward onto the person equal to his weight. When a string, cable or wire pulled taught transmits a force, it's called "tension." Applied force occurs when one object applies force to another such as a person pushing a box.