Beyond the pronouced hip-hop influence, Nu Metal has--like most forms of heavy metal music--proven somewhat difficult to define. Some fans and musicians have a firm concept of genre and subgenre, but others reject such categorization as unnessesary, limiting or useless. There is often significant crossover from one category to another, and often the influence of non-metal music.
Some heavy metal fans do not consider nu metal a form of heavy metal music at all, arguing the genre is too diluted from what they consider "true" heavy metal, further noting that nu metal guitarists typically forgo traditional metal guitar technique--especially soloing--in favor of tightly concentrated riffs. Other heavy metal fans, however, reject these arguments, citing rock music's long history of incorporating disparate elements--including jazz, experimental music and world music--out of curiosity or genuine appreciation for other musical genres.
The stage acts and video clips of some of the more commercially successful of these groups owe much to some of metal's more pompous traditions, without much of the menace that such stylings used to represent. This fact, combined with the supposed unoriginality and commercialization of nu-metal music, means that the style is often derided by other metal fans.
The commercialization and popularization of the genre is largely a result of the increased availability of aggressive music in mainstream outlets such as MTV. However, many would contest that rock and metal have grown in popularity of their own accord, perhaps with the aid of hip-hop influences. This also leads to the questioning of whether or not nu-metal is "true" metal. The bands of the genre tend to practice aesthetics that are unlike the metal community; favoring a lifestyle that is excessive (reminiscent to 80's hair metal) off of a teen angst basis. For its musical scale, it is conventional, simple, and radio friendly.
The genre is occasionally called "nü-metal," using the traditional heavy metal umlaut. "True" metal fans sometimes use the derisive term aluminium since nu-metal is "not as hard as metal and can be folded up and thrown away after use".
In the 1990s, many bands began to mix rapping and other new techniques with traditional heavy metal guitar and drum sounds. As a result, fans and music journalists needed to differentiate between the more traditional heavy metal music and this "new breed" of bands who were using samples, DJs, raps and drum machines in a way that made their music distinct. "New metal" evolved into the trendier spelling "nu metal," and a genre was vaguely defined.
Nevertheless, some distinction is usually maintained between rap metal, rapcore and nu metal. Rap metal is normally considered to be metal with primarily rap vocals -- with a minimum of other styles. Rapcore and nu metal are basically the same thing: distorted guitars and drums with rapinfluenced vocals. This means that it can sound somewhat like traditional metal vocals, with a varying amount of rap -- some bands more than others.
Nu-metal vocals can include rapping, clean singing, and screaming, sometimes all in the same song.
While traditional heavy metal was very much guitar-based, with intricate guitar solos and complex riffs forming an important part of most songs, nu metal groups tend to place more emphasis on other aspects of the music; the guitar is often much simpler, usually with no guitar solos or technically difficult riffs, but using harmonics and down-tuned strings to create a distinctive sound. Machine Head, on their 1994 debut album, Burn My Eyes, were one of the first bands to begin using this style of guitar, although they still incorporated guitar solos into their songs.
Traditionally, metal bass lines tend to be quite simple, often following the root note of the guitar riff. (There have been a few notable exceptions, such as Cliff Burton of Metallica, whose bass lines tended to be much more complicated, and in general the complexity of bass part varies with the style of each given band.) In nu metal, however, bassists tend to use more complex lines, often influenced by jazz. Limp Bizkit's Sam Rivers, for example, comes from a jazz background, as does their drummer, John Otto.
Nu-metal drumming is often influenced by the complex breakbeats of hip-hop. In fact, many notable nu-metal bands feature a DJ who provides sampled "beats" and other effects. The most famous of these are DJ Lethal of Limp Bizkit and Joe Hahn of Linkin Park.