head and shoulders

I know virtually nothing about TA but often hear about the head and shoulders formation as a negative signal which I think is apparent in the past few weeks on the ADO chart. If I am correct in recognising this shape does it give any suggestion as to how far the price will fall? Should the volume be considered as well as price? And most importantly, is there any explanation as to why this pattern works as a negative signal, or has is essentially become a self-fulfilling prophecy? Any information to help a novice is appreciated.