Can’t access from outside

I’ve just managed a successful new installation, thank all the deities, but I’ve hit a snag. My blog can’t be accessed from anywhere in the known universe. It’s based on my own server, as is my forum which anyone can access by going to http://myIP/forum/index.php and I would have expected the same for wordpress – http://myIP/wordpress/index.php.
It occurred to me that there may be an incorrect setting in Options. In both the following spots – WordPress address (URI) and Blog address (URI) is http://localhost/wordpress/index.php – should one (or both) of these be different?
Oh, and by the way – what’s the difference between a URL and a URI? Does anyone know?