Are there any differences among these addresses?
www.example.com would refer to the same thing. Modern browsers assume that the protocol is
http://. Some browsers are even starting to hide the implicit
http:// when showing URLs in the URL bar.
example.com can be very different. It is possible to run two different sites at those addresses with completely different content. That wouldn't be good for users and most websites take one of three approaches to making those addresses work as expected: