from Apple's Doc:
Target-Action in UIKit:
You set up a control so that it sends an action message to a target object by associating both target and action with one or more control events. To do this, send addTarget:action:forControlEvents: to the control for each target-action pair you wish to specify. When the user touches the control in a designated fashion, the control forwards the action message to the global UIApplication object in a sendAction:to:from:forEvent:message. As in the Application Kit, the global application object is the centralized dispatch point for action messages. If the control specifies a nil target for an action message, the application queries objects in the responder chain until it finds one that is willing to handle the action message—that is, one implementing a method corresponding to the action selector.
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