I've been on six Carnival cruises and have two more booked, one leaving in two weeks.
There are seven (actually eight) classes of Carnival ships:
Fantasy class - Fantasy, Ecstasy, Sensation, Fascination, Imagination, Inspiration, Elation & Paradise
Spirit class - Spirit, Pride, Legend & Miracle
Destiny class - Destiny, Triumph & Victory (actually the Destiny is a little different than the Triumph & Victory, thus the eighth class)
Conquest class - Conquest, Glory, Valor, Liberty & Freedom
Splendor class - the Splendor is the only ship in this class
Dream class - Dream, Magic & Breeze (the Magic is almost complete and the Breeze will not be finished for about one year).
I have not been on the Splendor or Dream.
The Fantasy class is made up of the oldest and smallest ships in the fleet. These ships are used for the three, four and five night cruises. There are no balcony cabins (the suites have balconies, but there are no category eight cabins). The lifeboats block the view from the Verandah deck. All of these ships except the Elation and Paradise have been upgraded with an improved pool area, an adult only Serenity deck and a water works. Actually the Elation and the Paradise have the adult only Serenity deck, but not the improved pool area and the water works (a miniature water park). However, it will be added to each ship in the near future. These ships do not have the optional extra cost steakhouse restaurant nor the Seaside Theater (a very large outdoor TV screen for showing movies and sports).
The Spirit class is my favorite class of ship because it is easy to navigate (because it does not have a mid ship dining room - both dining rooms are in the back of the ship and is in fact one large two level dining room), it is just the right size (not too big, not too small) and these ships have the biggest passenger ratio of all Carnival ship (meaning the most space per passenger). These are currently the largest Carnival ships that can fit through the Panama Canal. These ships have the optional extra cost steakhouse but not the Seaside Theater nor the water works. These ships also have one pool with a sliding roof (open in good weather, closed when the weather is not so good).
The Destiny class is more or less a 90% scale version of the Conquest class, except the that they do not have the optional extra cost steakhouse.
The Conquest class is about 10% larger than the Destiny class and includes the Seaside Theater, a pool with a sliding roof and the optional extra cost steakhouse. It does not have the water works. All Carnival ships (all 24 ships) have water slides, multiple pools, a casino, a main show lounge, smaller lounges, a dance club, game room (arcade) and several bars.
The Splendor is just a little bit bigger than the Conquest class (its sister ships are actually in the Costa fleet - Costa is an Italian line owned by Carnival). It has the optional steakhouse, water works and a pool with a sliding roof. I do not believe it has the Seaside Theater.
The Dream class is Carnival biggest and newest class of ships. These ships have it all, the steakhouse, the water works, Serenity deck, a pool with a sliding roof and the Seaside theater.