The keep is a web based application. It is much more flexible in terms of extending it to mobile application.
Web application, website, all the same thing to the user, just terminology. Not sure what you mean by "more flexible" I'm afraid, but regardless - the Keep may already be mobile-optimised, we don't know at this point. Seems like it would make sense, but I guess that depends on how exactly choices are presented and what kind of rich content accompanies them (videos, "codex" and background information, pictures at least - we just don't know right now). I don't see the Keep ever being rolled into an "app" (the downloadable, installable kind) - no advantage that I can see to that, since it needs internet connectivity anyway.