NSOperationQueue and GCD

NSOperationQueue and GCD

NSOperationQueue predates Grand Central Dispatch and on iOS it doesn't use GCD to execute operations (this is different on Mac OS X). It uses regular background threads which have a little more overhead than GCD dispatch queues.

On the other hand, NSOperationQueue gives you a lot more control over how your operations are executed. You can define dependencies between individual operations for example, which isn't possible with plain GCD queues. It is also possible to cancel operations that have been enqueued in an NSOperationQueue (as far as the operations support it). When you enqueue a block in a GCD dispatch queue, it will definitely be executed at some point.

To sum it up, NSOperationQueue can be more suitable for long-running operations that may need to be cancelled or have complex dependencies. GCD dispatch queues are better for short tasks that should have minimum performance and memory overhead.