What catagorizes nu metal as a genre?

The reason I ask this is because you see bands like korn and disturbed that are clified as nu metal when they have a completely different sound compared to bands like limp bizkit and linkin park, I'm just wondering if there is a similarity that I'm not seeing that clifies them all as one genre of music, I know bands don't all sound the same but it's rediculous to me to hear a band like korn play and hear linkin park when they don't sound alike but they are still catagorizes as the same genre.