PDT and IPN in Paypal

1. Read this: http://www.opencart.com/index.php?route ... h=32_35_60

2. Having IPN and PDT is referred to as the "Belt & Suspenders" method. The analogy meaning both items hold up your pants, if one should fail, the other should complete the job.
http://osrinivas.wordpress.com/2010/08/ ... mystified/

3. It does help for payment reliability and validity. Additional steps are still necessary for people using hacked email accounts as you have no control over who has access to a person's paypal account. The only way to verify then is to hold the order in "pending" and use your own email verification to have a customer click a link in their paypal email. This is something I'm looking into now, mainly for downloadable products

Yes you can use any url, even the store url

Yes if you enable PDT then paste the PDT token in your opencart paypal setting page and it will use it

PDT is a belt
IPN is suspenders

Both of them will work fine without the other.. it is just good to have a backup.
You don't "need" PDT.. but its good to have. You won't really notice any difference unless IPN can't reach your site, then without PDT to back up the process, you may find missing orders.