Make sure that you go through the booking process to the end to see what the final cost is. There are always taxes and port charges and the prices are per person. What is covered sometimes depends on which cruise line you are sailing.

On Carnival it will cover:

cabin with daily cleaning

all the food you can eat in the main dining room, buffets, pizza, sushi, snacks, iced tea, lemonade, coffe and juice at breakfast. It does not cover soft drinks, juice during the day, food and drinks in the stores on ship, or alchohol. Carnival also has free room service but you should tip the server with cash when they bring the food. I always order room service for breakfast since I can get smoked salmon this way since it is not available in the buffet.

You can buy a soda card for an additional charge to get soft drinks and juice from the bars. Carnival allows you to bring onboard soft drinks for your own consumption. You can also bring wine but there is a corkage fee for opening and consuming it in any dining place (drink it in your cabin for free).

Most ships have a specialty restuarant that has an additional charge if you choose to eat there.

entertainment -movies, shows, trivia, games, etc. There is a charge for bingo since it is for prizes.

Not included: internet - you can buy minutes or a package of minutes for use

soft drinks - you can buy a soda card or buy them as you want

alchohol _ there are often daily specials

tips - they add $10 per person per day but this can removed
if you choose (not recommended) or added to at the end
of the cruise

shore excursions - you can book through the ship, book on your
own, or just do nothing and hang around the
ship/port

laundry - there is a charge is you need to wash clothes

cell phone charges - you may or may not get service and it
could be very expensive using your US
phone internationally