The DDE is available only to Annual Passholders or Florida Residents and is basically good for 20% discount on food and beverages (including alcoholic) at most table service and buffet restaurants run by Disney. This includes the lounges in the resorts. It also includes the food courts in those resorts without a restaurant and Counter Service in Animal Kingdom.
Disney Dining Plan Is designed for people staying at a Disney Resort who are either paying full rack rate for their room and purchasing their ticket as part of a package or for DVC members staying on points. Each person three and older on the reservation must take the DDP ands it is for the number of nights of the reservation. You get one meal credit per person per day each for table service, counter service and snack.
One thing not mentioned above is the DDE is good for a year .. or 13 months depending on when you order it. If you visit WDW often you can use it for several trips.