Sometimes it's hard to distinguish between "Hard Rock", and "Metal", especially since the definition has changed over the years, but I'm taking my best shoot at the differences between the two. If you like heavy music, check out both generes.
AC/DC: Here's a band that proves you can not only rock, but rock some seriously fun party tunes with only three chords. Hide your diploma in the closet, crack a beer, and just have some fun with these boys!
Led Zeppelin: Usually credited as being the first "Heavy Metal Band", Led Zeppelin remains one of my favorite bands of all time (Though by today's standards they would probably be concidered "Hard Rock" rather than "Metal")
Rammstein: A very cool melding of Crunchy Heavy Metal Guitars with Industrial music. Dark and sinister, and hard not to bang your head to, Rammstein rocks!