Slipknot is nu-metal.
If it´s not heavy metal/power metal/thrash metal/speed metal, it isnt a classic....and even those are discussable. Some bands are not playing the 4 above, but still are classics, there are exceptions as always, but this is a slight guideline. What Parkwaydrive and avenged sevenfold, as I lay dying and bullet for my valentine are is core, be it metalcore/hardcore/emocore/grindcore/deathcore or whatever. Though some argue they could be considered screamo.

Screamo is not a genre, it's the lamens term for a set of vocal techniques. And isn't Linkin Park considered nu-metal?

Nu-metal is a very wide genre. But I have to agree screamo isnt really good. Im a hardcore fan from nyhc like sick of it all down to the newer ones like hatebreed(to name a big name :D ) and a lot of other shit.

Well, I am not a hundred percent sure, but I guess its this really highpitched screaming music with mostly emotional lyrics about losing your gf and such. But I dont know for sure.

Nope, it's NOTHING. It's what people that know nothing about vocal techniques say. I know people that think shouting is screamo. What you're refering to is shitty emo bands, but even they can use a range of vocal techniques that can be described as "screamo", that other good bands will also use. The difference is they suck at everything

Thank you for the education good man. Sorry, really didnt know that, just didnt want to cause a shitstorm by forgetting a word and someone coming to me and saying "Hey they are screamo you idiot...hurr durr" ^^