Concurrency means "running multiple tasks simultaneously". Concurrency allows iOS devices to HANDLE background tasks (such as downloading or processing data) while maintaining a responsive user interface. In iOS, you can manage concurrent tasks using Grand Central Dispatch (or GCD), and Operations (formally known as NSOperation). In order to achieve concurrency, iOS provides three WAYS as follows: 

• Dispatch queues: They are used to manage tasks in first-in-first-out (FIFO) order and execute tasks sequentially or concurrently. This is an easy way to handle asynchronous (not occurring at the same time) and concurrent tasks in your application.
• Threads: An independent sequence of instructions can be executed separately from other code within a program. Through threads, ONE can execute multiple code paths simultaneously in a single application. Having a thread is especially useful when you need to perform a lengthy task without affecting the execution of the REST of the program.
• Operation Queues: Operation queue OBJECTS are invoked in accordance with their priority and readiness. Essentially, operation queues are high-level abstractions of queueing models, built on top of GCD (Grand Central Dispatch). It is possible, therefore, to execute tasks concurrently, just like GCD, but in an object-oriented manner.