is the link to the gratuity guidelines on RCI's website. From what I understand, you can prepay your gratuities when you book, or have them added to your onboard expense account, or pay in cash.
Specialty restaurants are an additional charge. I think Portofino and Chop's Grill are around $20.00. Johnny Rockets, if your ship has it, is $4.95 all you can eat. Drinks are extra. Any bar drink you order, there is a charge for that, plus an automatic 15% gratuity.
You will have gratuity envelopes delivered to your stateroom on the last evening(s) of your cruise. If you prepaid your gratuities, either when you booked or onboard and had them added to your account, you will have gratuity vouchers as well to put in the envelopes.
You may see in the RCI section that people are referencing SeaPass cards. This card is your room key, and also serves as your onboard expense account. RCI operates on a cashless system, so any purchase you make, they will ask you for your SeaPass card, whether it's for gifts, massages, alcohol, etc.