A "guarantee" booking or a TBA (to be assigned) bookingis basically paying a price for a certain category of cabin with the chance that you may be upgraded. If you don't HAVE to have a specific cabin, you can book a guarantee (normally at a cheaper rate) for a certain level (say a balcony or oceanview) and you are guaranteed at least that category of a cabin. Then if the cruise is not full or you book early enough, they will upgrade you into higher category cabin of the same class (like a larger balcony cabin etc..)
We have done it a couple of times. We have always seemed to get at least a little upgrade.
If you are not that fussy about your cabin it works out. Just be sure to book at the lowest category you would be happy with in case you do not get the upgrade.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013