Exception: Carnival has several cabins which are classified as inside cabins but actually have a french glass door which allows light into the room (no balcony, but the door can be opened). Carnival also has some cabins that have a window, but because the window has an obstructed view (that means there is a railing or object in the way) it is listed as an inside cabin. Royal Caribbean has some cabins that have a window but look out over an inside promenade area. These are called "promenade staterooms".
All cabins come with basic amenities, such as the services of a cabin steward to clean your room and turn down the beds, soap and shampoo in the bathroom, individual climate control, etc. But certain categories of stateroom come with added perks. Suites come with a variety of extras and privileges, everything from priority boarding to in-cabin bar setups. Spa cabins will offer spa-related perks, such as yoga mats in the cabin or a fancy showerhead; concierge-level cabins will give you access to a concierge and niceties like afternoon canapes; and even solo cabins might offer extras, such as the use of an exclusive lounge. How do you want to be pampered on your vacation?
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.