The word cybernetics derives from "Kybernetes" which is greek for "steersman". Cybernetics is about control systems in general. This covers electronics (amplifiers), hydraulic (flow control), mechanical (Watt's Govenor), and natural systems (the ear, brain, Gaia etc) amongst many others. At the most general level it can also be applied to human relations and a host of other things. This is unlikely to bring any new insight but it is occasionally interesting to think of things in these terms.

To control something requires information about its current state and so much of cybernetics is about feedback. Consider an arms race, one side invents new arms, the other feels left behind and invents new more powerful arms, the first side sees this and invents newer arms, repeat until Ragnarok. An arms race is an example of positive feedback. The difference between the two sides could be considered the system error. That is to say what we want is politically stability and politically stability is a stalemate in this simplified example. Financial Inflation is another classic example of positive feedback: wages go up, prices go up, repeat.

Negative feedback is the way to control this type of runaway system. Instead of manufacturing more arms the weaker nation could have destroyed the extra weapons of the enemy, weakening them instead of strengthening themselves. Then the enemy would not have had to catch up. This raid could, of course, lead to a whole new kind of escalation but the principle is sound. (Some kind of disarmament treaty perhaps?).

Classically the cybernetic system is a block diagram that looks like:
               ______     | 
       +      |      |    v
  I -----0--->|  A   |----0--+----> O
        +^    |______|       |
         |     ______        |
         |    |      |       |
         +----|  B   |< -----+

A and B are multipliers, I is the input, O the output and D is a disturbance in or from outside of the system. In the arms race example the input is the arms of the enemy, the output is our arms, A and B are to do with how much better our arms are than the enemy's and how long it takes to manufacture them. A disturbance in this example could be a destruction of some of our arms stockpile or something similar.

Remember that this is a control system, not a process. The information that goes in and out is largely irrelevent to cyberneticists. Most of the time all we want is O = I or perhaps O = 10*I. The relationship is simple, cybernetics is about maintaining that relationship in the face of changes in input, changes in the components of the system (A and B) and disturbances. Another perspective on feedback is presented below:

              __O__            |                   |  
           __/     \__          \                 /
         _/           \_         \_             _/
        /               \          \__       __/
       /                 \            \__O__/
      |                   |

        Positve Feedback         Negative Feedback

A disturbance would be a to give the O a little push. Negative feedback would make the O come back to the center. Positive feedback would cause the O to run away down the slope. Positive feedback isn't all bad though. It is really good at speeding up the response of systems, it just has a few control issues. Consider:

              ___O_                    |
           __/     \_                 /
         _/          \_             _/
        /              \__       __/
       /                  \_____/

       Positve Feedback   Negative Feedback

The positive feedback area makes for speedy system response and the negative feedback are provides the control.

This write up doesn't even begin to scratch the surface of cybernetics, control theory or its applications which are many and varied. There are many books on cybernetics and control theory but I cannot recommend any one over the rest. This write up is the result of a course for which the lectures provided most of the content so my thanks go to the lecturer and not a book.

This information was gained from the lecture series and notes provided by R.J.Mitchell, Dept. Cybernetics, Reading University