When you check in, you will be asked if you want to set up a room charge. You will be asked for some sort of payment to back this up. This can be done with credit card, debit card, cash, travelers checks, or Disney Dollars. If you use a credit or debit card, you will NOT be immediately charged that amount and NO HOLD will be placed. They will "run" the card, however, only to see if it is an active account, but NO HOLD IS PLACED.
Your room key is now your room charge card, as well as (in most cases) your park tickets.
Each level of resort has a MAXIMUM room charge than can be set up at one time. For Values it's $500, for Moderates $1K and Deluxes $1.5K. These are UPPER limits -- you can set it up for any amount UP TO that amount. Not all CMs realize that the room charge doesn't have to be set up for the maximum amount. The CM who checked me in at POFQ last month knew this, when I asked to set up my room charge account for $800, backed by my debit card. But when I was approaching my limit and called to get that account charged and to set up a new one for an even lesser amount, the CM on the phone initially insisted that at POFQ they "only" set up room charges for $1,000. I asked her to check with her supervisor, and of course she came back on the phone and apologized for her error.
If you never reach your limit, your account will automatically be charged at time of check-out. If you reach your limit, your account will automatically be charged at that moment. If you have a $500 limit and currently have a balance of $490 and attempt to room charge a $15 purchase, your card will be declined. At that point, all you have to do is CALL YOUR RESORT and verbally authorize another room charge account. It is set up to be this easy -- you DON'T need to physically go back to the resort to do this.
You can physically go to your resort's front desk as often as you like to get a printout of your account activity. If you find you are near your limit and think you might spend past your limit soon, you may want to tell them to go ahead and charge your credit/debit card and then set up a new charge, just so save the momentary hassle of doing it later.