Yeh, just to expand on what MontyMon said ...
That handly little Key To The World card can do many things.
At very minimum, it is your room key AND it is your proof that you are a Disney resort guest (you'd need proof, for example, if you want to take advantage of Extra Magic Hours at the parks).
Many people have park tickets coded onto their KTTW cards.
Some people have the Disney Dining Plan, so they have food credits coded onto their KTTW cards.
Some people set up room charging privileges, meaning they can use their KTTW card to purchase items throughout the parks and resorts.
At check-in, at your resort's front desk, you can use a credit card or debit card or cash or travelers checks (and so on) to set up a room charge account on your KTTW card. Once you do that, you can use your KTTW card to charge purchases to that account. You can use your KTTW card for room charging everywhere you can use a credit card, which is almost everywhere ... every restaurant (both quick service and table service), every shop, even every snack cart in the parks and resorts. Downtown Disney has a lot of places that do not accept room charging.