Every time I peer at this thread someone has answered all the technical stuff already
NAT is a tricky thing, as said above all you need to know is you have two distinct networks, your house - Local Area Network (LAN: typically 192.168.x.x addresses) and the rest of the internet - Wide Area Network (WAN: everything else).
Your modem/router will have a WAN address which anyone on the internet can connect to. Network Address Translation (NAT) is the translation between WAN and LAN. So you need to tell your router how to take a WAN access and connect it to one of your LAN machines (in this case your PVR). This is done by port forwarding.
So say you access your WAN address with a web browser, it will connect to your WAN IP address on port 80 and effectively say "gimme a webpage".
If you have that port 80 forwarded to your PVR, then that router will pass it on to the PVR, so the PVR sees "gimme a webpage" and will return it to the router which passes it on via the WAN side to the web browser that requested it.
Sounds more complicated than it is!
So all you need is a rule that takes WAN port 80, and maps it to a LAN address also on port 80.
The only complication is that your WAN address can keep changing, which is where the whole dyndns type thing helps (gives it a name which tracks the WAN address)
I guess a lot of that has been covered above but figured a recap in different words might help
