I'm not sure why you got down voted to 0 for asking a legitimate question. The block objects are part of Grand Central Dispatch. A programmer tags an isolated unit of work as a block. The OS then takes these blocks and schedules them into execution queues. While the compiler adds support for defining the blocks themselves, OS X 10.6 provides purpose to these blocks. Just like you can have a VM which supports threads on an OS that itself is not multi-threaded, the feature would be pretty useless if only the compiler were updated as it has been for 10.6