background thread 종류

Creating a concurrent queue

let concurrentQueue = DispatchQueue(label: "queuename", attributes: .concurrent)
concurrentQueue.sync {

}  

Create a serial queue

let serialQueue = DispatchQueue(label: "queuename")
serialQueue.sync { 

}

Get main queue asynchronously

DispatchQueue.main.async {

}

Get main queue synchronously

DispatchQueue.main.sync {

}

To get one of the background thread

DispatchQueue.global(attributes: .qosDefault).async {

}

Xcode 8.2 beta 2:

To get one of the background thread

DispatchQueue.global(qos: .default).async {

}

DispatchQueue.global().async {
    // qos' default value is ´DispatchQoS.QoSClass.default`
}

If you want to learn about using these queues .See this answer