AP's are Annual Passes to the WDW parks. With them come some advantages, such as discounts on resort accomodations. There are a certain amount of rooms set aside in participating resorts that AP holders can get at a discount, depending on time of year and availability. The discounts have been know to go up to about 45% off of rack rate... sometimes, more.
Also there is a newsletter that you will receive quarterly, telling you what activities are available for Passholders, for instance next weekend, there will be a sneak peak at the newly revamped ToT where the park will close at regular time and reopen only for passholders to ride ToT and RnR. During the warmer seasons there are also passholder lounges in the parks where passholders can go and cool off.
Passholders have been afforded discounts on merchandise in some of the park's stores and restaurants as well.
Not only that, but having an AP saves you money if you happen to be going to WDW parks for 8 days or more during one year.
Hope this helps answer some of your questions!