Do you have to have a whirlpool bathtub or a walk-in closet? Will you be entertaining and thus in need of a dining table that can seat six or eight? Do you want benefits like priority dinner reservations and being first in line to get on or off the ship? Do you want to be pampered with extra-plush linens and bathrobes, fancy bath products and in-suite coffee and booze? You can find those amenities and more in most of the upper-level suites.
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.
Cabins are priced based on location on the ship. Typically cabins on lower decks are less expensive. Typically cabins midship (in the middle part of a deck) cost higher. Balcony and Suite cabins on the very aft of the ship can also be more expensive than other cabins.
Pay attention to the unique cabin setups on your ship, as they are not all created equal. Disneys four cruise ships, for example, have large standard staterooms designed to accommodate families. Even inside cabins may have a sleeping section that can be curtained off from the living area along with a split bath system. (One bathroom has the shower/tub and sink, another a toilet and sink.) Carnival is also known for having larger-than-average standard cabins, while Silversea, Regent Seven Seas Cruises and Seabourn ships feature all-suite accommodations. Norwegian Epic cabins sport the "new wave" design, with curvy walls and separate rooms for showers and toilets; sinks are located in the main cabins. As mentioned earlier, cabins at the very front and back of a ship often have different layouts than the cookie-cutter cabins that run the length of the ship.