For information on what metalcore is, go to the
hardcore metal section
here.
I'm not an expert, but here are some rough indicators on how I view different styles of hard rock and metal (those with relation to metalcore). See the previous link for information on the genres:
Hard rock with few metal influences (a lot of them came before metal became popular):
Hard rock with heavy metal influences:
(Traditional) heavy metal:
Thrash metal:
Metalcore:
Generic post-hardcore:
Original hardcore punk:
This list may be incorrect. Ask someone who has been listening to metal longer than me if you're in doubt.