Here's a thread on tipping. See my post near the end for the "everything you want to know about tipping.
Here's an excerpt that answers your specific question.
Your
Server (takes your food order and brings you your food),
Assistant Server (takes your beverage order, brings the bread to the table and assists the server in delivering your food,
head server (the person that makes sure that your server and assistant servers are doing their jobs. Also comes around on the last night and introduces them self and says "remember to tip me please

) and
Room Steward (Cleans up your room several times per day, brings you ice for your cooler, usually makes towel animals, turns down your bed and makes up the couch into a bed and sets up the pull down bed every evening. We've found these folks to be some of the nicest/friendliest people working for you.
Now for the how much (the following amounts are PER GUEST (including children) for THE ENTIRE cruise). This is from
DCL's web site.
What about gratuities? (September 2004)
We'll make it easy. It is customary to give gratuities in recognition of service, which you will most likely be treated to in abundance aboard the ship. As a guideline, may we suggest the following:
Per Guest per cruise______3-Night____4-Night____7-Night
......Dining Room Server___11.00______14.75______25.75
Dining Room Asst. Server____8.00______10.75______18.75
.Dining Room Head Server____2.75_______3.75_______6.50
..Stateroom Host/Hostess___10.75______14.50______25.25
Dining Manager Your Discretion
Room Service Your Discretion
For your convenience, a 15% gratuity is automatically added to bar, beverage, wine, and deck service tabs. All gratuities can be charged to your room charge.
At the Walt Disney World® Resort, 15% is the customary gratuity at restaurants and bars.
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This is me again to give a real world example
So if there are 3 of you on a four night cruise your tips would be:
Server________3*14.75 = 44.25
Asst. Server__3*10.75 = 32.25
Head Server___3* 3.75 = 11.25
Stateroom_____3*14.50 = 43.50
________________Total: 131.25
So here is how the tips break down PER GUEST
3 Night Cruise - $10.83 per day or $32.50 for the cruise
4 Night Cruise - $10.94 per day or $43.75 for the cruise
7 Night Cruise - $10.89 per day or $76.25 for the cruise
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For your specific situation - you're on a three night cruise, but I don't know how many people are in your party --- so I'll do this for 3 people for 3 nights.
So if there are 3 of you on a THREE night cruise your tips would be:
Server________3*11.0 = 33.00
Asst. Server__3*8.00 = 24.00
Head Server___3* 2.75 = 8.25
Stateroom_____3*10.75 =32.25
________________Total: 97.50