Hello, Nicola!
You are going to absolutely love the Dining Plan when you travel to Walt Disney World!
For your Table-Service meals, your Dining Plan does include tax, however it does not include gratuity for your server. Because you are a party of 4, you will not be automatically charged the additional 18% gratuity on your check, but will be able to leave the gratuity of your choice at that time. You may leave cash, pay by credit card, or even charge the gratuity to your room, if you've set up your Key to the World Card for charging privileges. So convenient!
For detailed information about the
Dining Plan, take a look at this very useful PDF, which includes a sample receipt; this will help you better understand what to expect when you get there.
For Quick-Service, tax has also been included in your cost, and there is no gratuity since there are no servers involved. (When we dine at our favorite Quick-Service restaurant, Wolfgang Puck Express, at Downtown Disney's Marketplace, servers DO bring orders to the table and refill Guests' drinks, so we do leave them a gratuity for taking such good care of us.)
Send more questions as you think of them! We love to help!