Death, Black, Thrash, even "nu" (urgh), different styles, different bands, different attitude. Well, I am a metalhead myself so I am more or less acquainted with the different styles these days. My preference is Swedish Death Metal, great bands continue to come out of Sweden, but there is excellent German Thrash and great Norwegian Black. When Metal started to differentiate or rather "spread" into these different styles, I know not. However, it all seems to have started when classic bands like Black Sabbath or Venom went on playing heavier music. It would be great to read a serious case study of the different styles of Metal and their origins but for the time being, we must content ourselves with listening to bands from all over the world that blend Metal with technical complexities from Jazz and/or classical music, which has given Metal a new fresh air. Please note that I'm not talking about bands like Dream Theatre which - despite being awesome musicians, I don't think they qualify as a Metal band, properly speaking, but rather progressive rock if you will. DT's "feminelli" vocalist has definitely nothing to do with Metal so... draw your conclusions. And that American "invention" so-called "nu-metal"... well, why bother really, it's crap made up by producers and the showbiz which American teens buy merrily, and so do other teens around the globe. If you ask me, that "thing" should be banned from record stores. 
But oh well, Regardless of the styles, Metal styles have one thing in common: unconditional followers. We can say that one of the main characteristics of Metal is precisely the fans. Once you get into Metal, you don't let go. As Aussie rock'n roll band
AC/DC put it brilliantly, Metal "shakes your foundation" and paraphrasing them too, Metal ain't no pollution, so let us keep on head banging to the overdriven guitar riffs, double bass drumming and infernal growling. \m/