GCD

All dispatch queues are first-in,first-out data structures.

Types of dispatch queues:

1. serial (as private dispatch queues)
2. concurrent (as global dispatch queue)
3. main dispatch queue

Advantage:

The most advantage is the simplicity of the work-queue programming model.Dispatch queues let you focus on the work you actually want to perfrom without having to worry about the threard creation and management. 

Getting the global dispatch queue:

The sysytem provides each application with three concurrent dispatch queues. These queues are global and are differentiated only by their priority level.

dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

Creating serial dispatch queue:

dispatch_queue_t queue;
queue = dispatch_queue_create("com.example.MyQueue", NULL);

Dispatch queues and other dispatch objects are reference-counted data types.