Registered mail is mail that gets sent with the post office making a recording of it but registered mail doesn't require a signature upon deliver to the person the registered mail is being sent to.
There's also no proof of delivery that the sender can track down very easy with registered mail but the post office would have a record that they delivered the registered mail to the person.
It doesn't guarantee that the right person got the registered mail though but it shows that it was delivered to the address you sent it too.
Certified mail is mail that is sent to the recipient with a signature required by the recipient and without that signature of the person the mail is addressed to the mail will not be delivered and then after around 7 to 14 days the certified mail will be returned to sender.
Also Certified mail is designated as such because the sender gets proof of delivery and billing and also the person who the letter is addressed to must sign for it, serving as proof it got to them okay.
But the registered mail doesn't require the signature that serves as proof that it got to the person properly.