NTR - stands for "Netorare," which is a Japanese term. Basically has 3 variations/flavors to it: Pure NTR, Netori, and Netorase.
As for the definitions of the terms, I'll just quote a reply I had a while back to someone asking the same question...
Edit: Just to add, the trouble with F95 tagging is that the NTR tag basically covers all three flavors, so sometimes there's confusion if a title is Pure NTR (the rage bait), Netori (for those who want to RP as a scum/Mister-Imma-steal-your-girl), or Netorase (the I'm-a-cuck-and-I-like-it route).
Also NTR discussion is usually flamebait material in threads, so whenever someone invokes it in a thread, things usually go south real quick, mods get involved, thread gets cleaned. So it's best not to dwell on it too much unless said title has the NTR tag (and even in such cases, threads like that still end up as one big shitshow).