A "PIN" is a unique code that is sent in an email or a brochure in your regular mail.
It entitles you to discounts on rooms or packages that can exceed those offered to the general public-it might me a question of dollar savings or dates for those savings that the general public cannot book or both. Sometimes the offers made by PIN eventually become offers made to the general public and sometimes not.
As an example, our upcoming trip was booked using a PIN we received in November that got dubbed the "Cinderella" PIN. At the time we got it the general public offer was for 40% off a "room only" reservation at a deluxe resort but it excluded the Animal Kingdom Lodge-AKL was offered to the public at 25% off. Our PIN got us 35% off at AKL, saving us some money. It also motivated us to stay at a deluxe resort as opposed to a value or moderate.
No one is sure exactly how Disney decides who gets what, but in order to put yourself in the running to receive PIN offers you should register for the Vacation Planning DVD at the Disney website. It is also suggested that you price out one or more hypothetical trip packages and "save" them in your profile.