1) nu-metal is an aggressive and raw subculture of metal which generally fuses influences from grunge, hip-hop, alt-rock, and electronic sounds usually with thrash and groove metal.
2) A name used by intolerant and ignorant fucktards who feel the need to slap labels on music in order to descriminate against things they don't like. Usually done by metal n00bs, pretenders and tryhards.
3) A kind of metal with lots of variety, all bands in the genre suffering from the bad reputation of few of them.
Slipknot, Korn, Limp Bizkit and Disturbed (as well as others) are often called so.
1) dude1: "Disturbed is pretty good, but some of their music is a bit strange."
dude2: "Yeah, Nu-Metal can be like that."
2) metalhead: "Man, I love Mudvayne, Pantera, Slipknot and Metallica."
cockfag: "lolzors. wtf wrong wit yo n00b? Slipknot iz totaly Nu-Metal. You r stoopid and ghey."
3) "Limp Bizkit is basically rap, but Slipknot is metal to the core."