http://www.example.com and 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.
www.example.com and 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:
Redirect example.com to www.example.com
Redirect www.example.com to example.com
Serve the same content at both example.com and www.example.com