Grindcore is a fusion genre. Mixing death metal and hardcore punk, grindcore pushes sonic limits. Some of the most known grindcore artists are Napalm Death (who are now more of a death metal band), Nasum, Pig Destroyer and Leng T'che. Napalm Death are generally credited with creating the genre. Their album, "Scum", is considered the first grindcore cd.
Grindcore is generally considered more abrasive then death metal. Some grindcore bands choose to go with a drum machine instead of a human drummer. These bands generally speed the drums up to over 1000 beats per minute. These bands, and other bands that include industrial elements, can be considered cybergrind. One of the best known cybergrind bands is Agoraphobic Nosebleed.
Grindcore songs are generally quick. Many songs are less then 2 minutes, and it is extremely rare to see a song longer then 3 minutes. One grind cd I own goes through 99 songs in 19 minutes.