Consider two people throwing balls to each other. Let the two people be A and B. When A throws the ball to B, he/she will experience a force due to the ball, and when B receives the ball, he/she will also experience a force due to the ball with the same strength that A have experienced, but in the opposite direction. Suppose they are now throwing a transparent ball and are standing on a skateboard. If there is another person, C, watching them throwing balls at this moment, then C will probably see A and B moving away from each other as if there is a distant force acting between them. (Remember C cannot see the ball because it is transparent) In fact, the action of the fundamental forces (except gravity) can be understood as 'gauge bosons' (basically, some kind of elementary particles or transparent balls in the ball throwing picture) being exchanged (thrown) between the elementary particles. Different forces can be interpreted as different gauge bosons being exchanged. For example, the electromagnetic force is the exchange of a photon, which is the particle representing light, and the weak force is the exchange of something called the Z0 or W+ or W- particles.