You can use the for anything you would charge to your stateroom. You just take your Disney Dollars to guest services and they apply them to your room as a stateroom credit. You can then use your room key to charge against your credit.
With a few small exceptions, the ship operates as a "cashless" society, so unfortunately you can't use your Disney Dollars to pay for individual transactions at the stores/spa/bars. It all must be charged to your room.