A classification used for a wide range of alternative rock acts, some of which have metal roots, some of which have rap elements, and some of which are really hard rock. This term was used by the media to define these acts back when they first started coming out after the alternative craze so they could be classified in the same genre, even though they don't really sound much alike. In reality, there is no such genre as nu metal, as every band that could be said to be in this genre really belongs to another. For example, Linkin Park is rap metal or rapcore (and really their latest stuff is just rock) while Disturbed is more Alternative Metal.
Metalheads will often complain about how much nu metal sucks but they really shouldn't because there is no such genre. Better stick to hating individual bands.
If you think Linkin Park and Disturbed are both nu metal, you're fucking insane.