OBC - On Board Credit. When you get on board you get assigned basically a charge account (backed with your credit card) that you charge everything you buy on board to. You room key is also your charge card...called your sign and sail card. OBCs are just credits that show up on this account. So they just reduce your balance at the end of the cruise. Unless they state otherwise an OBC can be applied to anything but cash advances in the casino (I think) but if you don't spend the entire OBC they don't refund the extra. But unless you get a huge OBC they normally don't amount to more than your auto tips and or you bar tab.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007