Caffeine is a drug (it has been banned in Burma as a narcotic) and as a result does have withdrawal symptoms for example the cells of the body have become over sensitive to adenosine as a result when you stop taking it there is less adenosine than required by the body as it takes time to re-equilibrate to the new conditions the result is a decrease in blood pressure which results in an excess of blood in the head which causes a headache.

The effects of an overdose of caffeine these are very unpleasant but probably won't kill you.
They are:
Flushed face
Gastrointestinal disturbance
Muscle twitching
A rambling flow of thought and speech and others.

However having said that it wont kill you, you can die from a large overdose Orally this about 7.5 grams. In cups of coffee this is about 50 to 200 cups. For more information see links.