Yeah, sure.
Heavy Metal is a much debated genre by itself. One reviewer of a Led Zeppelin album coined that term. It's been downhill ever since.
See, there are many subgenres, as well as sub-sub-genres... There are a ridiculous amount of sub-genres of Metal. Luckily for you, this is not Punk (which is even more intense in terms of quantity).
Heavy Metal = Iron Maiden, Motorhead, early Running Wild, early Black Sabbath. Traditional metal; blues riffs done with the key of a "satan's" Or "devil's" chord. One of the most early forms of offensive rock. Because people are stupid.
Heavy metal eventually decided to be faster.
Speed Metal = early Metallica, early-mid era Judas Priest, mid-era Running Wild, mid-era Tankard, Anthrax. Really just "faster" heavy metal.
Speed + Punk = eventually metal and punk decided to make some sexy little babies = Thrash.
Mid-Metallica, Megadeth, Slayer, Tankard, Kreator, Destruction, Sodom, Razor, and last but definitely least; Slayer.
Thrash created the whole concept of "extreme metal". Which eventually turned into Death and Black. Power Metal is divided into two sides; Blue collar USHM and Heavy Metal++. Basically continuing the normal aspects of "heavy metal".
Death is generally considered a direct evolution of Thrash. Black was a revolution against death, or metal in general. Death is Thrash++, whilst Black is Thrash+European stylings+Crust Punk.
Black Sabbath actually created four styles at once without notice. Heavy Metal, Stoner rock, Doom Metal and Sludge. Stoner-rock is blues-heavy metal. Doom Metal is similar, yet much slower and with more distortion. Sludge is stoner+doom+Orange amps. Fuck you, other metalheads, you know it's true.
Shit like Viking Metal and Folk Metal doesn't actually exist. They're just themes. Lyrical content does not equal musical qualities.
If I forgot anything, you guys should tell me. Jameson is not healthy.