PNR and reservation are interchangeable. Keep in mind, a lot of this terminology goes back to the days of paper tickets (which can still be used, BTW). The reservation was the note in the airline's systems that said "passenger XYZ will show up at the airport for these flights." The ticket was something that passenger XYZ would carry and present to the airline for boarding (hence the terminology "lifting the coupon" -- the gate agent would remove one individual ticket coupon from the book of four as payment for the current flight). Since the airline rarely sold its own tickets directly, the reservation was all the information that they had until they got the audit records back from the travel agent. A "confirmed reservation" is imprecise terminology: what we really mean is "a reservation with a valid e-ticket attached."

Again, this is industrywide SOP. When you go through the purchase process on United.com, you can request seat assignments prior to entering your credit card number -- because the reservation (PNR) is already active, despite the fact that no matching ticket has been applied to it yet.

It's less common now than it once was, but not all fares require immediate ticketing. For example, my understanding is that full-fare government tickets (YCA) often don't need to be ticketed until ~72 hours prior to departure. Some agency-booked tickets have similar rules. In these cases, the reservation is still perfectly valid, but it's not going to have a confirmed ticket attached to it for a long time.