The wall, poke and other nuances are facebookism for ways of communicating and publishing.  The wall is really just a public comment on someone else's profile.  It's a way of expressing yourself, subtly marketing something or give your friend a little love. 

You can climb it, throw things over it, maybe get out a little spay paint and do some wall art.   Just kidding.  Actually when you add the grafitti app you can spray paint on someone's wall.  And if you add the application for the super wall you can do pretty rich stuff like add a picture or video to someone's wall.  Have fun!

On the "walled garden" 

Facebook is a "walled garden" because the vast majority of Facebook is not publicly crawlable by search engines or the general public.   It's that whole damn "permission" thing.  Anyway, developers push what they can in terms of making interesting applications in facebook, but generally speaking people need to grant appropriate permission in order for data to be moved around or viewed in facebook.