I know what DPS and HPS is, but I haven't heard of NPS. I didn't see anything on the AMR site that defines what the "N" means in NPS.
It is a tanking metric that we created for Legion. This newer forum actually has pretty good search -- searching for "nps" will give you some stuff. The most relevant is this:
Thanks yellowfive, I did search the forums for "nps" and 0 results appeared
strange... you should get around 17 results: http://forums.askmrrobot.com/search?q=nps