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How does an underscore in front of a variable in a cocoa objective-c class work?

When creating a new project in Xcode 4, the boilerplate code adds an underscore character when it synthesizes the ivars in the implementation file as:

@synthesize window = _window;

or:

@synthesize managedObjectContext = __managedObjectContext;

Can someone tell me what is being accomplished here? I'm not a complete nube, but this is one aspect of objective-C I don't understand.

Another point of confusion; in the app delegate implementation, after synthesizing the window iVar as above, in the application didFinishLaunchingWithOptions: method the window and viewController ivars are referred to using self:

self.window.rootViewController = self.viewController
[self.window makeKeyAndVisible];

but in the dealloc method it's _window, or _viewController

Thanks

objective-c ios4 coding-style
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