NTR as a whole is when a player sees the action or gets a detail description later in the game. There are different types of NTR.
NTR A "The heroine is willingly cheating on the protagonist with another guy and enjoys every second of it. No rape, blackmail, drugs etc. involved, it's consensual sex from the very beginning."
NTR B " Initially, the heroine is raped, blackmailed, drugged or otherwise tricked to have sex with someone other than the protagonist. However, she eventually starts enjoying it and the sex becomes fully consensual in the end."
NTR C "he heroine is raped in front of the protagonist and he can only helplessly watch it. It's non-consensual from start to finish."
Protagonist-organised Netorare "This kind of netorare is orchestrated by the deliberate actions (or inaction) of the protagonist. The heroine wouldn't willingly cheat on the protagonist, but he intentionally manipulates the situation so that she will end up raped by other men, or just flat out convinces/orders her to have sex with them. Either he does this for personal gain (selling his girlfriend for money or using her to obtain blackmail materials on someone), wants to push the heroine away, or he enjoys such a situation where the heroine has sex with other men."
This information is taken from vndb.org. They have more types of the NTR and definitions.
P.S I cant post a hyper link due to the rules of this web-site. I am new here.