FB’s settings page seems to have changed drastically and the guides on net are outdated. Please refer to the screenshot below.

What does the “Site Url” and “Site Domain” stand for, as I have already provided my Canvas Url on another page and it seems to be working fine.
It’s for web apps and websites that integrate or interact with Facebook.
http://developers.facebook.com/docs/guides/web/
Site Url is the main URL of your website/ web app
Site Domain indicts which domain authentication is to be enable on for your application – typically this will be the same domain as your site URL