OK I'll try to explain. A Cat 12 room is the cheapest room you can get on a
Disney Cruise. They are basically the same room as Cat 11 (Cat 11 and 12 do not have split bathrooms) but they are on Deck 2 - and mostly under the nightclub (noisy). There are only 13 Cat 12 rooms on the whole ship (and all the rooms only sleep 3) Disney seriously overbooks these rooms - so if you book a Cat 12 you are most likely to get upgraded to at least an 11 (you are guaranteed an upgrade if you have 4 people.) As far as I know all Cat 12s are booked as GTY meaning you are guaranteed at least a Cat 12 but might get a higher Catagory. So what are you giving up by booking a Cat 12 GTY? Well, Disney has the right to put you in any room they have available (and they can change the room up until boarding- although this rarely happens). So basically it is a surprise where your room is going to be...I like surprises

and since we have 6 (I booked 4 people in 1 room, 2 people in the other) in our party it saved us several hundred dollars. I received 2 cat 10s, (lowest catagory with a split bath), right next to each other, unfortunatley right under the nightclub, but since I don't have small children I think it will probably be OK.
A break down of savings at today's prices using expedia:
Cat 12 GTY for 4 people $2596
Cat 11 for 4 people 3316
Cat 10 for 4 people 3416
Remember if you book 4 people in a cat 12 you are automatically upgraded to at LEAST a Cat 11. A savings

(in the above example) of $720. just to let Disney pick the room. That is a lot of fun money to spend on other things. You can book these rooms thru Disney or your
Travel Agent.
I should mention that sometimes, other catagories also go to GTY status - basically this means that the room catagory is very close to selling out so you will, at least, get this catagory (maybe higher) although a specific room number is not available and no discount is given.
Hope this helps,
Jennifer