Grindcore is a form of heavy metal that combines elements of death metal, industrial metal, and hardcore punk. The main characteristics of grindcore include heavily distorted and de-tuned guitars, very fast tempos and incomprehensible vocals. Its lyrics tend to lean toward socio-political themes as well as gore. Many grindcore songs are extremely short in length, gaining them the term ‘microsong.’
Early pioneers of the genre include Napalm Death, Siege and Repulsion who drew their influences from a variety of bands including Minor Threat, Black Flag, Venom, Motorhead, Dirty Rotten Imbeciles, Corrosion of Conformity, Slayer, Metallica, Septic Death and Sodom, among others.
Bands of this genre include:
Dance Club Massacre
Here are some links where you can read more about the genre:
Grindcore Wiki article
List of grindcore bands