Netori: This is when the protagonist takes someone else's partner. In other words, they seduce or start a relationship with someone who is already in a committed relationship, from the perspective of the one "gaining" the partner.
Netorare (NTR): This is when the protagonist loses their partner because they are seduced or cheat on them. The focus here is on the suffering or humiliation of the one who is "betrayed.
It's all cheating in the end...