Nu-Metal is a hybrid genre.
It combines the heavy rock genre 'metal' with different singing styles ie rapping. A perfect example of the is 'The Truth' by Limp Bizkit. Another good example is 'Spit It Out' by Slipknot. Within these bands there is also a m'ixer' who will provide sounds that cannot be created using convential instruments. Within the convential instruments guitars are often tuned down to give 'heavier' riffs. to compliment this the drummers will use the bass drum more often (and will usually have two bas drum pedels)
Pre-Nu-Metalers often look at the new genre with disregard as it is not ture rock music.
Korn, Slipknot, Limp Bizkit and Perfect Circle are all examples of Nu-Metal music.