The basic Emacs key-bindings should work in all Cocoa apps. For instance, the URL bar of your web browser, while text editing in just about any editor (maybe even in Office, but I have not tired in a while), etc. Basically, anywhere you have a cursor, the basic line-editing key bindings should work.
So, in practical terms, this means you have 1 + n clipboards[1]. The main global GUI clipboard (using the command key), and then an app-specific clipboard (using the control key with Emacs bindings). As a developer, it is extremely helpful to have two clipboards in TextMate, for instance.
[1] Technically, the clipboard in OS X maintains a history (not sure what the limit is), but the OS X GUI only exposes the last cut/copied item.
So, in practical terms, this means you have 1 + n clipboards[1]. The main global GUI clipboard (using the command key), and then an app-specific clipboard (using the control key with Emacs bindings). As a developer, it is extremely helpful to have two clipboards in TextMate, for instance.
[1] Technically, the clipboard in OS X maintains a history (not sure what the limit is), but the OS X GUI only exposes the last cut/copied item.