To get the plan you pay $35 per day per adult and $10 per day per child. It is added to your total trip price and paid up front before you arrive. There is a list you can look at on WDW website as far as which restaurants are available to you to get your meals in - tons to choose from. You are allotted 1 table service meal (TS), 1 counter service meal (CS), 1 snack per night of your stay. You must at least have a 1 day base ticket into parks to qualify for dining plan add on to your stay.
IE: five nights x 1ts, 1cs, 1snack per day = 5ts, 5cs, 5 snacks per per for the length of your trip.
The price does not vary according to what you order again 35 adult per day/10 child per day. You tell the server at ts that you are on the dining plan and they will let you know if there are any exclusions on the menu at that location. Table service is appetizer, entree, dessert, and nonalcoholic beverage for each person in your party. If there is no child menu your child can order off adult menu, also if no appetizer, dessert on child menu they are allowed to order those items off the adult menu as well.
CS - easiest way to explain would be like McDonalds, Burger King, Kentucky Fried Chicken, etc. Fast food - not necessarily burgers. Your counter service meal includes a combo meal - sandwich, fries, drink and also a dessert. There are of course variations of this - salad, drink, dessert for example. Also Cosmic Rays in MK has 1 CS meal that would be 1/2 chicken, ribs, mashed potatoes, and a vegetable - you can add a drink and dessert and this is 1 CS meal so it doesn't HAVE to be a burger and fries type thing. You have to go to a pickup window type deal to order these meals vs sitting down and having a waitstaff serve you.
We are going on the dining plan and find it well worth the money as it also included character meals that I wanted to do as well.