If you cancel before penalties begin, you will get a refund of any fare you have paid, refunded to the original method of payment. So, if you use gift cards for payment, be sure to keep those cards, and if you use a credit card, keep the credit card active. (I have heard it is possible to get a refund by check if you don’t have access to the original payment method, but that would add complexity and time.)
As auntlynne said, if you used a special offer attached to your booking, such as a placeholder booked onboard or an onboard credit from Disney because of issues with a previous cruise, you will lose that offer if you cancel. You can modify your booking by moving it (and some or all of the money you have paid) to a future date if you are fairly sure you will still cruise with Disney.
One more note: for Concierge, inaugural sailings, and Restricted fares (for which you must pay in full immediately), penalties begin as soon as you book. For a standard booking, you can cancel with a full refund prior to the penalty date no matter how much you have actually paid.