Originally the term referred to nu-metal bands, such as Linkin Park, Limp Bizkit, Korn, Papa Roach and Slipknot. More recently the term can be applied to screamo, emo and metalcore bands such as Bullet for my Valentine, The Used, Avenged Sevenfold and Senses Fail.
The defining feature of such bands is that they make angsty, commercially-orientated hard rock targeted at "rebellious" teenagers.
26-year-old metalhead: "Fuck off kid, you don't know shit about metal! You just listen to some bullshit mallcore. Power metal, speed metal, folk metal, black metal, death metal, thrash metal... those are all real metal genres. The shit you listen to isn't even close!"
14-year-old kid: "Jeez, man, thanks for the lecture you fucking nerd!"