Inside cabins with no views at all are typically the smallest, cheapest cabins onboard. They are great options for budget-minded travelers who do not intend to spend a lot of time in their stateroom, or who want to sleep all day in absolute pitch dark. They are less ideal for cruisers prone to seasickness, those who need natural light and groups who require a lot of in-cabin space. Not everyone will be happy in an inside cabin; it is worth upgrading if the lack of light will put a damper on your vacation.
Exception: Royal Caribbean and other cruise lines will also use sub categories to distinquish between how many occupants the cabin will sleep. Therefore a cabin that can sleep up to 4 people will be a different sub category from a cabin that sleeps only 2 people.
Do you need a balcony? Cruise travelers who spend all their time in the public areas -- sun decks, lounges, restaurants -- or on shore may be perfectly happy with standard-size cabins and no private outdoor space. Those who love to avoid the crowds and lounge quietly on their own verandahs or have private room-service meals outdoors will surely want balconies. Do not forget to take your itinerary into account; on a chilly-weather cruise, you might not be spending too much time outside, so depending on how much space and light you need, a balcony might not be worth the splurge.
If you tend to get seasick, cabin location is really important. It is a question of engineering, really. The lower and more central you are in a ship, the less roll and sway you will feel. Even if you choose a balconied stateroom, choose the lowest level and the most midship one you can find. The higher decks and cabins at the very front (forward) or back (aft) of the ship will rock and roll the most.