what method will called when we start to rotate device and after it finished

From Apple Docs:

Sent to the view controller just before the user interface begins rotating.

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration

Sent to the view controller after the user interface rotates:

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation

See more here: UIViewController Class Reference -> Responding to View Rotation Events

ATTENTION:This is deprecated, see this post

For newcomers to this post, the methods suggested by Nekto have become deprecated in iOS 8. Apple suggests to use:

-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator

You can use the "size" parameter as an easy way to get whether it is transitioning to portrait or landscape.

i.e.

if (size.width > size.height){    // Position elements for Landscape}else{    // Position elements for Portrait}

More info is availablein the Docs.

All the above methods(in the answer by @Nekto) are deprecated in iOS8.0 and later versions. Source: iOS Developer Library

As of iOS 8, all rotation-related methods are deprecated. Instead, rotations are treated as a change in the size of the view controller’s view and are therefore reported using the viewWillTransitionToSize:withTransitionCoordinator: method. When the interface orientation changes, UIKit calls this method on the window’s root view controller. That view controller then notifies its child view controllers, propagating the message throughout the view controller hierarchy.

In iOS8 or later you can use the below method.

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id <UIViewControllerTransitionCoordinator>)coordinator{    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {        // Stuff you used to do in willRotateToInterfaceOrientation would go here.        // If you don't need anything special, you can set this block to nil.    } completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {        // Stuff you used to do in didRotateFromInterfaceOrientation would go here.        // If not needed, set to nil.    }];}