CATOBAR is an acronym used in categorizing aircraft carriers. It refers to the type of flight operations those ships can support. CATOBAR stands for CAtapult Take Off But Assisted Recovery. Specifically, it indicates that the ship in question is able to launch heavy aircraft using a catapult, and that it has hook-and-barrier systems to safely 'arrest' them on landing.

This is in contrast to STOBAR or STOVL carriers. Although carriers which utilize CATOBAR must be larger, more complex and more expensive than STOBAR or STOVL carriers, it allows them to operate a wider range of aircraft. Since their aircraft do not need to be VTOL or STOVL designs, they can be simpler; and since the catapult provides additional thrust while the arrestor gear permits faster and heavier landings, heavier/larger aircraft can be used than would be possible otherwise.

tanktop asks an excellent question, which, paraphrased, is why is the 'But' in there? Wouldn't the 'b' stand for Barrier? Well, I have two answers for you, but don't claim I'm 100% correct, so if you have seen this acronym expanded with 'Barrier' please do tell me so I can investigate. First answer - the B doesn't stand for barrier, the point is that there is some on-ship recovery system which aids in arresting the aircraft. What type is not important. The second answer (from a Navy type I know) is that the 'B' is there because it was put into STOBAR, where the 'but' is a direct contrast.

As of late 2010, there are three nations operating CATOBAR carriers:

United States
Nimitz-class
USS Enterprise
France
Charles de Gaulle
Brazil
São Paulo