Multi Thread:

 Threads are one of several technologies that make it possible to execute multiple code paths concurrently inside a single application. Although newer technologies such as operation objects and Grand Central Dispatch (GCD) provide a more modern and efficient infrastructure for implementing concurrency, OS X and iOS also provide interfaces for creating and managing threads.
Multi threading can be clearly analysed by the below shawn image

Threads are relatively lightweight way to implement multiple paths of execution inside of an application.
Threads let you perform more than one action at the same time and they executes independently one of another.

These kind of task can be achieved by in two ways
NSOperation and NSOperationQueue
GCD (Grand Central Dispatch)


If you create an NSOperationQueue from scratch as in:
NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
It will be in a background thread:

Operation queues usually provide the threads used to run their operations. In OS X v10.6 and later, operation queues use the libdispatch library (also known as Grand Central Dispatch) to initiate the execution of their operations. As a result, operations are always executed on a separate thread, regardless of whether they are designated as concurrent or non-concurrent operations
Unless you are using the mainQueue:
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
You can also see code like this:
NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
[myQueue addOperationWithBlock:^{

   // Background work

    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        // Main thread work (UI usually)
    }];
}];
And the GCD version:
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
        // Background work
        dispatch_async(dispatch_get_main_queue(), ^(void) {
            // Main thread work (UI usually)
        });
    });

NSOperationQueue gives finer control with what you want to do. You can create dependencies between the two operations (download and save to database). To pass the data between one block and the other, you can assume for example, that a NSData will be coming from the server so:

    __block NSData *dataFromServer = nil;   
    NSBlockOperation *downloadOperation = [[NSBlockOperation alloc] init];      
    __weak NSBlockOperation *weakDownloadOperation = downloadOperation;
    [weakDownloadOperation addExecutionBlock:^{
     // Download your stuff      
     // Finally put it on the right place:      
      dataFromServer = ....     
     }];

      
    NSBlockOperation *saveToDataBaseOperation = [[NSBlockOperation alloc] init];
    __weak NSBlockOperation *weakSaveToDataBaseOperation = saveToDataBaseOperation;     
     [weakSaveToDataBaseOperation addExecutionBlock:^{
       // Work with your NSData instance
       // Save your stuff
     }];


    [saveToDataBaseOperation addDependency:downloadOperation];
      
    [myQueue addOperation:saveToDataBaseOperation];
      
    [myQueue addOperation:downloadOperation];
Note : Why I am using __weak reference for the Operations, can be found here. But in a nutshell is to avoid retain cycles.

Download example source code from here .