synchronous asynchronous background main thread and queue (usually just use "queue" as variable name, descriptive names below only for clarity)

threading

// get global queue
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// create serial background queue
dispatch_queue_t newQueue = dispatch_queue_create("com.domain.app.queuename", 0);

// dispatch asynchronously to a specific thread
dispatch_async(queueName, ^{
    // the slow stuff to be done in the background
});

// dispatch synchronously to a specific thread
dispatch_sync(queueName, ^{
    // synchronous stuff
});

// to use the main queue
dispatch_async(dispatch_get_main_queue(), ^{ ...
dispatch_sync(dispatch_get_main_queue(), ^{ ...