The recommended tip (which is added to your sign-and-sail card ) is $10 per passenger per day. It covers your cabin steward and wait staff in the formal dining room. You can add (or subtract) to this amount to reflect the degree of service you feel that you have received (do this at the Purser's desk at the end of the cruise).
NOT included is the %15 added to beverage orders at the bar, poolside, etc. Also not included is a tip for room service delivery (a few bucks here ensures speedy delivery next time!) Your Maitre'd also receives tips, and from this board, I have learned that most people base his tip (if any) on above-and-beyond personal service. I also follow this philosophy. Read more posts on this board and you will learn a LOT!
Good Luck!