Darn, I'm in deep water with demand curve. Why is the graph change along the movement but not shift or vice versa ?

Suppose the price of chicken goes down. What will happen to Fred's demand for chicken ?

http://www1.esc.edu/personalfac/fmen...ics/demand.htm
This says this is not a change in demand - it just a change in amount Fred will buy.

Generally
Fred will change his demand along the demand curve with a change in price, holding all else constant.
If there is a change in other things, such as the price of a complement/substitute good, or research shows that chicken is good for you but red meat isn't etc. Then the whole demand curve will shift for a given price point and he will demand more/less depending on the direction of the shift in the demand curve

Next one, shortage & surplus. Shouldn't shortage the price rises up bcos the quantity demanded > supplied. In reality this happens always, right ?
Same goes to surplus