I dont really understand about countersunk screws?

Question:I am supposed to learn about countersunk screws for technology class! The sheet I have just doesnt give me enough info on it. Does anybody know any good websites that can help me out?

Answers:
there are 2 reasons to counter sink screws. 1. in order to hide it from plane view some peopel prefer to see onl;y the wooden( or whatever material) finish of a job. sometimes the screws you have are not long eneough to gete a good grip on the other member you are screwing into so to get more lenght out of the screw you counter sink it.