A genre of music that developed in the mid 80's, when punk groups such as Siege and Sore Throat began to take their respective sounds to progressively further extremes. Repulsion is considered by many to be the first "proper" grindcore band, as their 1986 demo (later released on CD as the album "Horrified" in 1989) provided the listener with all of grindcore's key ingredients: noisy, fuzzy bass, blast beats, snarled vocals, short songs (often under two minutes) and minimal guitar work which relied on repeating, hyptnotic rhythms which were similar to hardcore punk but had a notable "metal edge." In the following years, several bands release important albums that further defined the genre: Napalm Death's "Scum" (1987), Carcass's "Reek of Putrefaction" (1988) and Terrorizer's "World Downfall" (1989). These bands presented a punkish sound that was excessively noisy and chaotic, generally lacked melody and was among the most extreme music of its time. Lyrically, they generally either focused on extreme blood and gore or social/political issues. By the 1990's, grindcore began to rise in popularity, with some bands being more punk or metal than others. Among the more important bands from this era include Anal Cunt, Impetigo, Pig Destroyer, Extreme Noise Terror, Dead Infection, Regurgitate and Last Days of Humanity among others. The more significant bands from the 2000's include the likes of Fuck...I'm Dead and Agoraphobic Nosebleed. A number of different sub-genres began to appear during the 90's, the most popular being goregrind, which was perfected by Impetigo. Another popular sub-genre that has developed a cult following in recent years in cybergrind, sometimes called techno grind, which combines grindcore with industrial and dance music, and is often low-fidelity and in many cases is produced by a single person. Other notable sub-genres include noisecore and powerviolence, which are often not directly associated with the metal scene (with the later often being more associated with the hardcore scene). In the last few years, bands such as The Locust and Dillinger Escape Plan have presented a sound that appears to have "elements" of grindcore (such as blast beats, unintelligible vocals and choatic guitar playing), but they tend to lack the punkish, traditional elements of the style, leading to debate over whether or not the bands should fit under the label.
Just because a band is noisy or uses blast beats does not mean they are grindcore. Like many metal genres, it is defined primarily by the guitar playing.