Both are unique identifiers, but a PNR can have multiple etickets associated with it and can have multiple passengers. PNR's are airline specific and a given journey with multiple carriers will have different PNR's associated with it.

I have also had PNRs reticketed by AE multiple times when there are changed. The PNR stays the same but gets new etickets.

So in short not all that similar. The PNR is a total record of your journey for multiple passenger by a single airline while an etickets number is a globally unique representation of a coupon for your ticket that will be used for at least part of that journey for only one passenger.