A genre of music that is meant to be pretty simple. Many grindcore bands use basic riffs at inhuman speeds and write songs ranging from 30 seconds to 2 minutes. The genre was started by Napalm Death and Carcass during the mid eighties. It later evolved to have a bunch of its own subgenres.
Goregrind which is grindcore influenced bands with gorey lyrics, the vocals almost always seem to use effects. Some goregrind is: Lymphatic Phlegm, Regurgitate, Squash Bowels, and more.
Pornogrind is usually very simple and contains a lot of erotic lyrics or odd fetishes to make the genre more humerous then serious. Some pornogrind is: The Meat Shits, Cock and Ball Torture, Anal Bleeding.
Political grindcore, Crustcore, noisecore are also some other grindcore sub-genres. But i myself don't listen to any of those so i can't really tell you what exactly they're like and what bands fall under that catagory.
Key Grindcore artists: Napalm Death, Carcass, Gut, Impetigo, Pig Destroyer, Nasum, Lymphatic Phlegm, Regurgitate, Morbid Savouring, Meat shits, Neuro-Visceral Exhumation
Slipknot is not nor anywhere near grindcore. The idiot who posted that obviously has no clue what the genre is if he seriously thinks that. Don't listen to his words, they are pretty useless.