$_get reads variable through a url,
$post reads the variable from a post method, ie from a form.
For example this url ishttp://phpbuilder.com/board/newreply.php?s=&action=newreply&threadid=10218425
might read the thread id variable through the get method.
Whereas, to reply to this thread you are "posting" information, so if the confirmation page where to mention the thread id, it would be from $_post.
Hope that is clear.
I am sure there are many threads on this subject throughout this forum.
Jamie