Um, this is a technical stuff. Hoping you know what I am talking about.
"rthk.org.hk" is the orginal stuff of registered domain name. The 'www' are the prefix of it which is set by the web administratrator (WA). Most of the WA set the direct all the links of original domain name (e.g. baby-kingdom.com" to their web servers "www.baby-kingdom.com". So it seems to be no difference for most cases. But "rthk.org.hk" does not do that by her WA. It requires your password and user name because "rthk.org.hk" link to their pirvate remote network. Their web server is sit on "www.rthk.org.hk". That's why you will see the difference of these two links. So the WA can add 'abc' for 'abc.rthk.org.hk" to form another private network for other functions. You can find more information at http://www.www.org (it is very very boring technical stuff for internet standards)