NTR is 'someone's someone is sleeping with someone else' but it has come to be an umbrella term for any kink, fetish or content where monogamy or 'fidelity' is at play, whether it's infidelity, swinging, cheating, cuckolding, swapping, whatever. 'Netorare' has many co/sub-sets, like Netori (MC does the SO stealing) or Netorase (MC willingly lets the SO sleep with others).