One of the big questions in booking a cruise is deciding which cabin to book. There are 3 main options: an inside cabin, an ocean view cabin, and a cabin with a balcony. Our family has, for various reasons, alway chosen an inside cabin. (Full disclosure: on a couples only cruise, we had an ocean view. We have never had a cabin with a balcony.) An oceanview cabin is on the outside of the ship with a porthole to see the ocean, or a balcony. Across the hallway are the inside cabins, on the interior of the ship. With an inside cabin, you do not have a view of the ocean or anything, really. (exception is the Magic Porthole, more below).
Why do we typically choose an inside cabin? Well, first, it is the cheapest. You get all of the amenities of a cruise, for less money. Second, the outside or ocean view cabins, tend to have small portholes anyway. It's not like you're getting that much of an ocean view (unless you have a balcony). Plus, when our kids were smaller, I worried about having a balcony -- which someone could fall off of! But probably the biggest reason we choose the inside cabin is that on cruises we are barely IN the cabin. We are too busy enjoying the activities on the ship or ashore, using the cabin only for sleeping and getting changed/cleaned-up. So for us, it doesn't make sense to spend extra money on a view that we will barely make use of anyway. Plus, if we want to see the ocean, all we have to do is go somewhere that has a window, or up on deck. I would recommend looking at the square footage of the cabin you book -- to me, that makes more of a difference than the porthole or lack thereof. It will vary from ship to ship, but sometimes you can get slightly larger cabins on the inside or a split bathroom (2 sinks, toilet and 1 sink separate from shower and the second sink.)
When you book a cruise, you should be able to see exactly which cabin you are getting! Look around and see what looks best for you. In addition to choosing the type of cabin you want, you can also choose which part of the ship and what floor it is located on. For a smoother ride, somewhere more towards the middle of the ship is best. Definitely keep this in mind if you or a member of your group is prone to motion sickness. Do you like being up higher? -- then choose an upper deck. If it doesn't matter, you may be able to get a larger cabin on a lower level. If you will have children at the kids' clubs, you may want to be on a level closer to where they are located.
Magic Porthole. Some inside cabins on the newer Disney cruise ships (Dream and Fantasy) have what's called a magic porthole. This makes it look like you have a real porthole and it shows you a view off the front of the ship. From time to time, Disney characters also appear on it. It's cute and interesting. The magic porthole shuts off at night, so it doesn't disturb your sleeping.