This is all absolutely correct:
There are 2 ways you can link to the second page, either with the absolute url (eg href="http://some/web/address") or with a relative path (eg href="../images/picture.png"). The relative path is relative to the page with the link in it. .. goes up one folder. ../images goes up one folder then into the images folder. data/websites/index.html points to the file index.html inside the folder websites inside the folder data, all relative to the current directory. If you put a / at the front of the path it becomes an absolute path relative to the root folder /
... BUT ...
1. The term "link" has two meanings.
a) If you mean an HTML "link", the above is true.
b) If you mean an OS "link" (as in "ln -s", for example)
... then while the above is still true
... it might not work UNLESS you allow "Follow symbolic links" in your http.conf file
2. If you mean HTML "link"
... then the path is NOT the physical path on your server's hard drive
Rather, the root ("/") is the "DocumentRoot" you specified in your httpd.conf file. The relative path is with respect to your "DocumentRoot".
Does it help with the problem?
Please keep us posted.