Hard Rock incorporates a number of elements, other than guitars. The definitive Hard Rock group is probably Black Sabbath. They have the guitar element down with Tony Iommi who played some of the heaviest, fastest and 'beautiful' riffs in rock and roll history. Take the minute long improvised solo during Iron Man between minutes 3 and 4 for a good example. Ozzy Osbourne is truly the epitome of a hard rock lead vocalist. He does not sing, he wails. If he's not wailing, he's screaming. Ozzy plays the crowd like only a few other lead vocalists can do. Rouding out the line up is Geezer Butler providing the heavy bass lines and Bill Ward supplying the necessary beat.

But Black Sabbath is not the only fine example of Hard Rock. Led Zeppelin is another very fine example. Once again Robert Plant provides the truly incredible vocals (deciding who is better, him or Ozzy, usually depends on how much of the whine factor you can take). Jimmy Page is quite possibly the greatest rhythm guitarist of all time (though my personal preference is Pete Townshend). They probably had an even better rhythm section than Black Sabbath with the horribly underrated John Paul Jones on bass and John Bonham on drums.

But this is not what makes a band hard rock. hard rock is truly a combination of the Blues with wailing vocals a very, very hard rhythm section and finally the kind of lyrics you'll find in Iron Man and Stairway to Heaven (IE: nonsensical).

So, Korn and White Zombie might qualify under the hard rock category, but they by no means define the category. That was, fortunately, done long ago.

A short list of good Hard Rock Bands:
Black Sabbath
Led Zeppelin
Iron Maiden
And not quite Hard Rockish but very close:
The Who
Jethro Tull
Rush