People who call Disturd and Slipknot hardcore are the first to be shot by me. Those fucks are Nu Metal.
A hardcore band is often more underground (not publicly known) than mainstream music. The music is much "purer" and more real than mainstream shit. The artists actually have a passion for what they do. Their music is their life, not their job. Most members of hardcore bands do it as a side pursuit, as they all have real jobs by day. One of my fave bands
Burnt by the Sun, although actually grindcore, has a WAN engineer for a drummer. The sad thing is most of the bands don't last that long. The somewhat upside to this is that they will put out a few excellent albums before their particular additions to the hardcore scene get old.
Examples are:
Every Time I Die
All Else Failed
Eighteen Visions (older stuff is best)
Evergreen Terrace
in reverent fear
Poison the Well