Just like when you go to a restaurant, certain positions on the cruise ship are paid a lower wage and are considered tipped positions. It is difficult when your food is paid for already to figure out what amount of gratuity should be given so most cruise lines have a recommended tipping amount. The positions usually tipped are your stateroom host/hostess, server, assistant server and head server. The recommend amounts will be between $9.75 per person per night to $14.00 or more per person per night. I use this site to figure out the amount recommended http://cruisetip.tpkeller.com/.
If you choose to prepay gratuities, the amount you will be charged is the recommended amount by that cruise line. You can add more when you get onboard or adjust to less if you feel the service was not up to par. The nice part to prepaying gratuities is that you do not have to fill out paperwork or go to Guest Services or the Purser's desk during your cruise. On cruises where tipping is recommended but not required, your servers and stateroom host will not know that you did prepay gratuities until your tip slips are delivered to the rooms. On cruise lines where gratuities are automatically added or included in the cruise fare, they obviously do know that you already paid the gratuities. Some guests will argue if the staff knows that they are getting the gratuities, they don't try as hard. I'm still undecided on this. I have prepaid gratuities a number of times with great service, but then I also have received less than excellent service on other sailings where the gratuities were prepaid.
When you order beverages onboard (when there is a charge), a gratuity is automatically included. This can range from 15-18%. Even though the gratuity is added, there is usually a space where you can add an addition gratuity if you wish. This can be confusing so double check your receipts when you sign them to make sure the gratuity was already added.