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.
47 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!