Hi Celestina!
You can get the latest information on the Disney Dinging Plan and the various options
here. There are over 100 Quick Service and Table Service restaurants participating in the Disney Dining Plan. For the most part, a person on the plan can order anything off the menu according to what the plan provides. The standard 2010 Disney Dining Plan allows you to order one entree, one dessert, and one non-alcoholic beverage at a Table Service restaurant; guest ages 3 through 9 can also order an appetizer.
I am not aware of any restrictions. However, in years past when it was possible for an adult to select both an appetizer and an entree at Table Service restaurants, there were some issues or misunderstandings that guests would run into from time to time. For example, at the Wolfgang Puck Cafe in Downtown Disney, sushi is considered an entree and not an appetizer. Some guests, including me, would want to order sushi as an appetizer and still pick an entree. Under the 2010 plan that is no longer an issue since appetizers are not included for adults.
Remember that even if you are on the Disney Dining Plan, you can still order additional items off the menu. So if there is an appetizer you want to try, you can just pay for it.
Enjoy planning your magical vacation!