To purchase the DDE card you must either be a FL resident($75) or an AP holder($50). The discount is good at almost all WDW owned full service restaurants even Victoria and Albert's. It is also good at counter service spots in resorts that don't have a table service restaurant (values and POFQ). You can also use it at all the lounges. It provides 20% off including adult beverages. An important new exception after January is CRT. The
DVC discounts are typically lunch only and this is anytime.
The dining plan provides one table service meal, one counter service meal and one snack (fairly limited choices I think) for each person in your party/per night. It is a flat cost per night. You can use the credits any day of your stay though so if you do two TS one day and two CS the next, it's all good. For DVC it sounds as if there are no ticket requirements but each member of your party must be on the plan. Gratuity is included at TS restaurants. There is lots more info on the dining plan on the restaurant board. There is even a FAQ about it. The price next year is going to be around $38/adult/night.
Have fun planning.
