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".
Very few ships actually have cabins dedicated to solo travelers. These will have sleeping space for one and can be quite small. The studio cabins on select Norwegian ships are the most famous example of this: The 100-square-foot staterooms each contain a full-size bed, nifty lighting effects and a large round window that looks out into the corridor. If you are a solo traveler, you will want to price out the cost of a solo cabin (usually somewhat higher than the double-occupancy rate of a similarly sized stateroom) compared to the cost of paying the single supplement (an extra fee tacked on if there are not two people in a cabin; the price can come out to as much as double the regular rate) for a standard cabin. And book early, as solo cabins sell out quickly.
Since cruising has become a popular family vacation, more new ships have built "family accommodations" into the actual design. These are often suites, each with a separate room for the kids -- sometimes a small alcove with bunk beds, sometimes an entire adjoining cabin. Families and groups can also take advantage of regular staterooms with third or fourth berths found in pullout sofas or pull-down bunk beds (called Pullmans). If you are going to squeeze your whole troupe into one cabin, make sure the space is big enough to accommodate the lot of you ... and all your belongings.
If scenery is important to you, take a good look at your cruise itinerary before selecting your cabin, specifically if you are choosing an outside or balcony. On a roundtrip Caribbean cruise or a transatlantic crossing, for example, the side of the ship you are on does not really matter. If, on the other hand, you are doing a one-way sailing (such as a southbound Alaska cruise or a trip from Barcelona to Rome), you might want to consider choosing a cabin on the side of the ship that faces the land. Sometimes the views can be breathtaking, and you would not get those views from the cabins that face out to the open sea.