A vastly overrated genre that has it's origins with crappy shoujo anime and manga. It's basically a Japanese term for cuckolding, and only a jealous mangina would do that to another guy.
The direct opposite is called Netori where the protagonist steals the girl from another guy, however only a loser would have that happen to him anyways.
Like most shoujo manga it usually but not always involves rape, is poorly written, cliched, done mostly for shock value,and takes little talent and writing skill.
While it used to stick to mainly hentai and shoujo, there a attempts at making it main stream but it the non existent plot is painfully obvious to those who aren't teenage girls or jealous old farts. Most artists and writers who use it tend in their works tend to be talentless hacks.
Only emotionally unstable teenage girls and obese former high school jocks like Netorare.
literally means "cuckold" and shortened as NTR is a hentai genre where in a heroine will be introduced as having a significant other, which may be a husband, a boyfriend, or even a partner in a BST affair. The story will then show the aforementioned heroine being intimate with another man thus provoking jealousy in the audience by proxy.