Hopefully (but not necessarily) one that is independent of language or framework?
there's a simple model at: http://en.wikipedia.org/wiki/Model-view-controller
you can find a uml diagram of the model and view here: http://www.dofactory.com/Patterns/PatternObserver.aspx. the controller is sometimes in the gui.
there are some flavours of mvc: http://en.wikipedia.org/wiki/Presentation-abstraction-control, http://en.wikipedia.org/wiki/Model_View_Presenter