Well the Castlevania theme isn't metal of course. It's a metal version I guess. I'd call it traditional heavy metal. I usually hate the "metal cover" Youtube videos that have soulless, modern metallish sounding riffs recreating songs. This is actually quite okay.

The genres become quite easy once you know something about the evolution.

Corrosion of Conformity had different phases.

Back when Mike Dean (and the other guy whose name I don't know by heart) were the vocalist they were Crossover thrash. To make it simple: Thrash metal (think Metallica) + hardcore punk.

Starting with "Blind" they changed music (and vocalist).
That album had elements of groove metal.
To make it simple: Groove was derived from thrash metal. Focuses on groove (obviously). Think Pantera.
It also had elements of sludge metal.
To make it simple: Just listen to the b-side of Black Flag's "My War". This was highly influential on future sludge metal bands:

Black Flag's "Nothing Left Inside" + "Three Nights"
https://www.youtube.com/watch?v=8a8eyCBQ1sI

Their next album "Deliverance" started their still current sound. With Peppar on vocals. Basically stoner metal with southern rock elements.
Stoner metal ... just think psychedelic rock + metal.

A good example of stoner metal:

Sleep
Dragonaut
https://www.youtube.com/watch?v=2-FjO3E8K-E

Does this remind you of Peppar era Corrosion of Conformity in any way?

Blue Cheer
Doctor Please (1968)
https://www.youtube.com/watch?v=tjbDzwjoQwA

Otherwise just call them heavy metal.

One of my friends keeps saying I need to listen to "Djent" - It's supposed to be some form of distorted math-metal i think

https://www.youtube.com/watch?v=dQw4w9WgXcQ
Here's a you tube link of every ones favorite^

https://www.youtube.com/watch?v=uBW5iDvkMHU find something that's not perfect in this video

pro tip: you can't

For death metal I'd add:
Cannibal corpse- Gallery of Suicide.

Nile- Ithyphallic

Dying Fetus- One shot, One kill

Lamb of God- For your Malice

Necrophagist- Only ashes remain

Decapitated- Spheres of madness

Death- Spiritual healing


Black metal

Malcuidant- Vestigen de Tradition

Gorgoroth- Sorg

Burzum- Erblicket die tochters des firmament

Dissection- The Somberlain

Mayhem- Freezong moon

Immortal- Call of the winter moon

Abyssic Hate- Bloodletting


And then there's

GWAR- Battle Lust

Who is a strong independent woman who dont need no genre


Idk what you'd call this, but I think it's good enough to make the list

Ufomammut- Idolum (full album)

Electric Wizard-Dopethrone

^theyre trying really hard to be sabbath, but at least they do it well

It's gojira


NEXT

Djent is like meshuggah. Just 2 or 3 notes played in a weird timing

GWAR falls into its own category. We all agreed on that?

depends on the sql column...

http://i.imgur.com/gw2mPoe.jpg

Ugh lol