Having been on a number of cruises, with different lines this is a REALLY difficult question to answer, as what I think will Almost always be different to the opinion of others so with the big caveat that whilst I stand by this answer, it might be considered personal, but that it also comes from a uk based person.
When it comes to quality most of the major cruise lines could be classed as 4 or 5 star, but Azamara, Silverseas, Seaborne, and Oceana, all claim to be six or seven star with added service, smaller passenger numbers, and routes that are different from the mass market lines.
When you look at the mass market lines such as those mentioned by the original poster, the more money you spend, the better experience you will get. The basic cost of a cruise gets you a small inside cabin, with no window (porthole). These are usually comfortably appointed with beds, storage, ensuite bathroom, and maybe a small seating area, more money gets you bigger cabins, a view, or even a balcony. Plus even more money - get even bigger, exclusive dining, butler service and much more.
Your price will get you all your food, entertainments, and travel to various ports.
You will pay extra for your drinks, excursions, and speciality dining. Also not included on most cruises are daily charges called gratuities by the cruiselines. These are usually added to your shipboard bill automatically.
Particularly aimed at families is of course Disney cruises.
Cunard, and Holland America tend to be be targeted at the top end of the market, with Princess, and Royal Caribbean, just below. Aimed at a more lively crowd is Carnival, which markets itself at 'fun ships', and Norwegian Cruise Lines.
Aimed at uk customers is P and O, Thomson, and Fred Olsen. Austraila has its own P and O, with Germany having Tui, and Spainish speaking markets having, Pulmantur, Italian speaking Costa, and Msc.
There are also other markets which I don't have any information on such as the far east.
Also each cruise seem have a different flavour, so joining a cruise that departs on a spring break week, will seem entirely different to a trip on the same Itinerary departing a few weeks later.
If you are gay you might want an lgbt specialist organiser such as Atlantis or rsvp.
My top tip is to pick your destinations, then try to match your cruise lines. If it's your first trip, I suggest finding a specialist cruise travel agent and having a conversation with them.
I haunt a cruise forum called cruise critic (other forums are available!) where you will see very lively discussion on the pros and cons of all the lines
Berlitz do a great annual guide to cruising which I found very useful before my 1St cruise.
And finally every cruise I have been on has always cost me more that I originally planned for, beware the bar bill at the end of the cruise.