ios – What’s the distinction between a dispatch queue and a completion block?

Can anybody please clarify the distinction between a dispatch queue and a completion block? For instance, there’s such a dispatch queue as:

dispatch_queue_t concurQ = dispatch_get_global_queue(DISPTACH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurQ, ^{
     dispatch_sync(concurQ, ^{
         [someObject someHeavyTask];
     });
     dispatch_sync(dispatch_get_main_queue(), ^{
         // run another process
     });
});

And there’s such a way name:

dispatch_async(dispatch_get_global_queue(), ^{
            [someObject someHeavyTask completion: ^{
            // run some other task
        }];
});

Is there any distinction between these two mechanisms? Thanks.