Nu Metal (sometimes called aggro metal or nü metal) is a subgenre of Alternative Metal that emerged in the mid-1990s and is influenced by styles such as Funk Metal (Korn), Groove Metal (Soulfly), Death Metal (Slipknot), Rap Metal (Limp Bizkit) and Pop (Linkin Park), among others.
Nu Metal music can emphasize atmosphere through the use of pedals and rhythm over melody and complex instrumentation; often, Nu Metal songs use rhythmic, syncopated riffs, played with distorted electric guitars tuned in low ranges to create heavier music than traditional music, often making use of seven-string guitars to accomplish this purpose.
The band Korn is the pioneer of Nu Metal because they made such a sound of Alternative Metal mixed with the rhythmic Funk Metal making use of seven-string guitars and a five-string bass, resulting in a sound both rhythmic and heavy on their 1994 debut album Korn when no one else was doing it, becoming the inventors of Nu Metal.
The band Slipknot was a very important band for Nu Metal because they showed the most aggressive side influenced by Death Metal; their first two albums, Slipknot and Iowa, are the most influential for this side of Nu Metal.
And finally, the band Linkin Park with their two albums Hybrid Theory and Meteora created a school of Nu Metal more focused on Pop, being very influential for the development of the genre during the 2000s.
It is called Nu Metal because it comes from "new metal", because the press and certain producers believed that this was a totally new sound for Metal (though it hasn't evolved so much since the late 2000s). .