is content locking different from ppd?

i would appreciate if more light is shed on the difference between ppd and content locking. Than you.

The principal is the same but PPD usual means completing a survey or offer to access a file to download usually hosted on a PPD site and content locking means locking content via a gateway until completion of a survey or offer from the visitor to access content or videos on your site.

You can lock a specific part of your website in content locking, I believe it is not possible with PPDs.
Also, in Content Locking, since the content on your website is locked, you have the power to control lock settings, such as lock type- if you want to keep it has a forced lock (content will not be shown till the time survey is completed) or a timed lock (lock disappears after some time even if survey is not completed).

Content locking is where a specific part of a page are locked from users, and users must complete certain requirement such as a survey or answer few questions in order to continue of what they are doing.