There are three types of a queue available for GCD.
Main Queue : the main queue runs on the main thread, and it performs the serial execution of the thread.
Global Queue : The Global queue is a concurrent queue and shared by the whole system. There are four global queues that operate on different priorities. The priorities are high, default, low, and background, where the background priority queue holds the minimum priority.
Custom Queue : the developer creates the custom queues. They can be either concurrent or serial.
