NSResponder is one of the fundamental classes in Cocoa. Any class that can respond to events like key presses or menu commands should be a subclass of NSResponder. Each responder keeps track of it's "next responder", and each window keeps track of the object that's currently the "first responder". When an event happens in a window, a message is sent to the first responder. If that object handles the message, great. If not, it passes it along to its next responder. This is known as the "responder chain."
Normally, you don't mess much with the responder chain in Cocoa. The first responder is mostly determined by user actions, such as clicking on a control.
