What's the difference between the "global queue" and the "main queue" in GCD?

Among some other ways, there are these two ways to get queues in GCD:

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_get_main_queue();

If I'm not completely wrong, the "main queue" is executing on the main thread and is good for "callback" blocks which execute UI work.

Does this mean a "global queue" is one that runs on a background thread?

The main queue does indeed run on the main thread like you say.

The global queues are concurrent queues and from the main page for dispatch_get_global_queue:

> Unlike the main queue or queues allocated with > dispatch_queue_create(), the global concurrent queues > schedule blocks as soon as threads become available ("non-FIFO" completion order). The global concurrent > queues represent three priority bands: > > • DISPATCH_QUEUE_PRIORITY_HIGH > • DISPATCH_QUEUE_PRIORITY_DEFAULT > • DISPATCH_QUEUE_PRIORITY_LOW > > Blocks submitted to the high priority global queue will be invoked before those submitted to the > default or low priority global queues. Blocks submitted to the low priority global queue will only be > invoked if no blocks are pending on the default or high priority queues.

So, they are queues which run on background threads as and when they become available. They're "non-FIFO" so ordering is not guaranteed.

The 5 queues (4 background, 1 main) all have different thread priorities (-[NSThread threadPriority]) too:

                            -main- : 0.758065
      DISPATCH_QUEUE_PRIORITY_HIGH : 0.532258
   DISPATCH_QUEUE_PRIORITY_DEFAULT : 0.500000
       DISPATCH_QUEUE_PRIORITY_LOW : 0.467742
DISPATCH_QUEUE_PRIORITY_BACKGROUND : 0.000000

(tested on an iPod 4th gen and the simulator on a MacBook Pro)

Yes. You can run code like this on a device to test it:

dispatch_async(
	dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
			NSLog(@"Block 1a");
			NSAssert(![NSThread isMainThread], @"Wrong thread!");
			NSLog(@"Block 1b");
		});
dispatch_async(
	dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
			dispatch_async(dispatch_get_main_queue(), ^{
					NSLog(@"Block 2a");
					NSAssert([NSThread isMainThread], @"Wrong thread!");
					NSLog(@"Block 2b");
				});
		});

Global dispatch queue :

1. Tasks in concurrent queue executes concurrently [background threads]
2. Tasks are still started in the order that they were added to the queue

Main dispatch queue :

1. Available serial queue that executes tasks on the application's main thread.
2. It is usually called from a background queue when some background processing has finished and the user interface needs to be updated.