Heavy Metal isn't hard to understand. It really is broken down into 5-6 subgenres with sub subgenres below that which play with each style.
Power Metal: Iron Maiden and Judas Priest; focusing on heavy guitar riffs, long vocal holds. Typically called British Power Metal.
Symphonic Metal: Nightwish, Within Temptation and Stratovarious; focusing on orchestral sounds, with a type of vocal that sounds similar to something you'd hear in an opera.
Death Metal: Napalm Death and Cannibal Corpse; Lower tempo, lots of deep sounds, not always easily understood.
Alternative Metal/Nu-Metal: Limp Bizkit, Korn; A blend of rap and metal, popular in the late 90s which usually involved a calm tone most of the song followed by a fairly heavy chorus.
Thrash Metal: Metallica, Slayer and Megadeth. Fast music, Fast vocals. Like they have somewhere to be.
I personally love your classic metal: Ozzy, Iron Maiden and Judas Priest; though for the past 15 or so years I've really grown to love bands like Nightwish, Within Temptation and Stratovarius. They break from the traditional mold of what we think of as Metal and focus on really good music and wonderful lyrics, showing it isn't all heavy.
Metal really is the most versatile music genre incorporating pretty much every other style of music and just bumping it up to 11. I'm not really a fan of Cannibal Corpse, but they have their fans.
I do like linkin Park, korn and Metallica
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