Quoted from allearsnet.com
"These little cards are literally your Key to the World.
A card with a magnetic strip on the back is issued at check-in to each member of your party. It is your room key and also identifies you as a Disney resort Guest. Always carry it with you as you may be asked to show it to gain entry to the parks on Extra Magic Hour days, or even to board Disney transportation. In addition, if you purchased Ultimate Park Hopper ("length of stay") tickets or certain other admissions media, these can be encoded on your cards as well, and used in the turnstiles at the parks, water parks and Pleasure Island for admission. If you wish, you can have charging privileges added to one or more of your cards by leaving a credit card number on file at your resort. This will enable you to charge purchases, activities and meals back to your room account. I love this option because I don't have to carry lots of cash around with me"