Donna has said it very well.
But the best advice is to book your cruise through a travel agent, not through the cruise line! If you have any questions or need anything, your agent will be right there to help you with all the details. Their rate will be exactly the same as booking it directly with the cruise line and often they will have better rates or amenities not offered by the cruise line. Plus, if you have any problems and booked it with the cruise line, who do you think they represent? An agent represents you and are there if you need anything.