Cocoas First Responder, an irl investigation of terms

List of terms/sentences from an article on the Responder Chain that I don't understand yet.

- In fact even Apple often employs delegate protocols to have a view controller communicate to its superior.
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@protocol FlipsideViewControllerDelegate

- Now we can get rid of the delegate protocol

Oh interesting.  There are 2 ways to assign the receiver for an action (like a button click).  One is to assign it to the "File's Owner".  No idea who this is yet, but sort of get the idea that it may be something called a "Controller" class.  Or something.  The other option for an action target is that you can put the action (the click) in a 'queue' of sorts, to be processed by the "Responder Chain" (whatever that is).  The chain goes up a list, sort of like the View Hierarchy in Android, until it finds a receiving method (I don't think that's what these are called but whatevs) that is registered for that action (the click!).  That method is then called.  

- Any selector can be used for the action parameter of the sendAction method 

What's a selector?  If I had to guess, I'd say that it's a view?

Since the app delegate has been promoted to a UIResponder as of iOS 5

Who's the app delegate?  This seems like the first Activity from an Android app.  Or like an Application subclass, but more easily accessible than the Application subclass is. 

Further Interface Builder questions:

There are three items listed under Placeholders: File's Owner, First Responder, and Application.  What is Application good for?

What's the difference between the Placeholders objects, and the Objects?