> What exactly is this program downloading? 
Httrack makes a mirror, a snapshot of a site/page

> Does it include the php code of the page? 
You can not get php, cgi, sql or any other server side code from the public
site of a web server, only html and related files.

> Is it possible
> someone to download and edit my site, then to
> release a clone one?Of course they can, they don't need httrack to do it
either.
<http://www.httrack.com/html/abuse.html#WEBMASTERS>