Most cruiselines auto enroll you into their loyalty program (don't worry it's completely free). At their core, all these programs are the same; the more you cruise, the higher your membership level. The higher your membership level the greater the rewards.
Unlike hotel and airline loyalty programs, points don't expire, and you don't need to complete a certain amount of cruises, or nights to maintain your current membership level. Once you "level up" you stay that level for life.
Loyalty programs do differ between the companies but you can typically expect complimentary drinks, laundry, casino free plays, cocktail parties, upgrades, and more. Check with your cruise lines individual program for specific details. Note that some items need to be picked up or claimed, so I recommend checking which benefits your membership level provides before you board.