There are actually three different Disney Dining Plans, and they all work a bit differently.
If you're referring to the most common "basic"
Disney Dining Plan (
DDP), a typical table service (TS) meal includes an entree, a dessert, and a non-alcoholic beverage. (Table service credits can also be used for buffets, family-style meals, etc.) At some restaurants, you can order from a "fixed price" sub-menu that throws in an appetizer as well, but limits choices on entree and dessert to the lower end of the menu. You can always order additional items (appetizers, alcoholic beverages, etc.) and pay for those items out-of-pocket. You cannot exchange a dessert for an appetizer under any circumstances.
The "deluxe" plan (DxDP) offers a bit more. First, you get three meal credits per night, each of which can be used for TS or counter service (CS) at your discretion (basic DDP includes 1 TS and 1 CS credit per night). Secondly, you get an appetizer at TS meals (lunch and dinner only) in addition to the entree, dessert, and beverage.
The quick service plan (QSDP) provides 2 CS credits per night -- a CS meal includes entree or combo, dessert, and beverage.
One of the areas of confusion is that the basic DDP lists that kids' meals include an appetizer, entree, dessert, and beverage -- but a kids' "appetizer" isn't anything like and adult appetizer. It's easier to think of a kids' meal as an entree plus two items, coming off of menu sections nebulously labeled "appetizer" and "dessert". (We frequently swapped in a fruit cup "dessert" as the kids' "appetizer", for example)
