The buffet is included in your cruise fare..... many food options will be offered on the buffet for 3 meals a day and dont forget the 24 hr. Pizza area as well ! There is a dining areaq within the buffet areas and is very casual. Many folks choose to eat at the buffet as they just do not want to go into the traditional dining rooms for their meals. For the most part, most passengers will use the buffet mostly for breakfast and lunch, but the dining room is a nice change for those meal times to avoid the crowds and to enjoy a more "country club" style lunch.
I would suggest that you also check out carnivals website as there is a wealth of information and photos (including ones specific to your ship) that will probably answer alot of your curiosities, so as to be better prepared for your upcoming cruise plans.
Of course you can get personal opinions/experiences here too !