If I tell you the URL, I'm passing by reference. You can use that URL to see the same web page
I can see. If that page is changed, we both see the changes. If you
delete the URL, all you're doing is destroying your reference to that
page - you're not deleting the actual page itself.
If I print out the page and give you the printout, I'm passing by value.
Your page is a disconnected copy of the original. You won't see any
subsequent changes, and any changes that you make (e.g. scribbling on
your printout) will not show up on the original page. If you destroy the
printout, you have actually destroyed your copy of the object - but the original web page remains intact.