With that in mind, I decided to post this "sticky" post explaining the different "classes". I hope this clarifies the situation.
Now, let's meet the different HAL ships, starting with the smallest and oldest:
PRINSENDAM - Built 1988 - 37,983 Gross Tons - 793 passengers
The smallest and oldest HAL ship, she is in a class by herself. She generally does the longest and most expensive cruises. She is the only HAL ship not built for the line, she was built in 1988 as the ROYAL VIKING SUN, became SEABOURN SUN in 2000 and then PRINSENDAM in 2002. Among things that separate her from the rest of the HAL ships - she does not have a two-deck dining room or main lounge, an indoor/outdoor pool, or dedicated children's facilities. Her smaller, more intimate size and off-the-beaten path itineraries are appreciated by her increasingly loyal following.
STATENDAM - 1993 - 55,810 GT - 1,251 pax
MAASDAM - 1993 - 55,451 GT - 1,251 pax
RYNDAM - 1994 - 55,819 GT - 1,251 pax
VEENDAM - 1996 - 55,451 GT - 1,251 pax
These four ships are the "backbone" of the HAL fleet. They are virtually identical except for color schemes and artwork. They were the first new HAL cruise ships to have features like the atrium, the two-deck high dining room and main lounge, and the indoor/outdoor Lido pool. These very versatile ships have done everything from 7-day Caribbean and Alaska cruises to full World Cruises. Recently they have been upgraded with features like the Pinnacle Grill alternative restaurant and the Neptune Lounge concierge lounge (for suite passengers).
ROTTERDAM - 1997 - 59,652 GT - 1,316 pax
VOLENDAM - 1998 - 60,906 GT - 1,440 pax
ZAANDAM - 1999 - 61,396 GT - 1,440 pax
AMSTERDAM - 2000 - 61,484 GT - 1,380 pax
The R-class ships are very similar to the S-class, but a bit longer and wider. Unlike the S-class, they are not all identical. The first of the four was ROTTERDAM. Specially designed for longer cruises, she is the fastest ship in the fleet. She also introduced new features like an alternative restaurant, concierge lounge, and Internet Cafe (all since retrofitted to the older ships as well). VOLENDAM and ZAANDAM followed; they are similar to ROTTERDAM but slower (the same speed as the S-class ships) and are a bit larger because the aft pool was moved up one deck, creating more indoor space on the deck below. Unlike ROTTERDAM, they were designed for yeoman duty in the Caribbean and Alaska though they also are suitable for longer cruises. Like the S-class ships, only decor separates these twins. The final R-class ship, like AMSTERDAM, like a hybrid of ROTTERDAM and VOLENDAM/ZAANDAM. She is faster than VOLENDAM or ZAANDAM but not as fast as ROTTERDAM. Like ROTTERDAM, she was built specially for longer voyages. ROTTERDAM and AMSTERDAM, together, are considered HAL's "flagships" and along with PRINSENDAM they usually do the longest and most prestigious cruises - the World Cruise along with the various Grand Voyages.
ZUIDERDAM - 2002 - 81,769 GT - 1,848 pax
OOSTERDAM - 2003 - 81,769 GT - 1,848 pax
WESTERDAM - 2004 - 81,811 GT - 1,848 pax
NOORDAM - 2006 - 82,318 GT - 1,918 pax
These are the biggest and newest HAL ships currently in service. They were designed mainly for shorter (less than two weeks) cruises in the Caribbean, Alaska, and Europe. Like the S-class, they represented a departure from previous HAL ships in size and design. While bigger than HAL's other ships, they are still much smaller than some competitors' ships which are now sometimes bigger than 150,000 GT and can carry over 3,500 passengers (Royal Caribbean have now ordered a ship that will be 220,000 GT and carry over 5,000 passengers). These ships are notable because of the exceptionally high number of private balconies - over two-thirds of cabins have them. They are called the Vista-class because they are named after the points of the compass in Dutch - Zuid (South), Oost (East), West, and Noord (North). The first three are identical; NOORDAM incorporates a few design changes which make her larger and have a slightly higher passenger capacity.
This will be two (or more) ships which will be larger, improved versions of the Vista-class, much as the R-class can be considered improved versions of the S-class. Right now there are few details but they will probably be about 90,000 GT with a passenger capacity of about 2,000.