Let the Map of Metal guide you
Web designer Patrick Galbraith has partnered with heavy metal historian Nick Grant to create the most addictive website I've encountered in weeks. The Map of Metal presents a vast network of hard and heavy subgenres, with brief but informative contextual details and a hearty YouTube-powered playlist to familiarize yourself with each subset of shred.
It's a fun and fascinating way to get a quick tutorial in a style of music that often seems too vast and splintered to tackle. Well, quick is a relative term; browse the landscape for a few minutes and see if you don't get sucked in. I'm currently discovering the wonders of Diamond Head in the New Wave of British Heavy Metal section. Are they evil? Yes, they are.