Black Flag was an American punk rock band formed in 1976 in Hermosa Beach, California. The band was established largely by Greg Ginn, the guitarist, primary songwriter and sole continuous member through multiple personnel changes. They are widely considered to be one of the first hardcore punk bands.

Black Flag's sound mixed the raw simplicity of the Ramones with atonal guitar solos and frequent tempo shifts. The lyrics were written mostly by Ginn, and like other punk rock bands of the late 1970s early 1980s, Black Flag voiced an anti-authoritarian and non-conformist message, in songs punctuated with descriptions of social isolation, neurosis, poverty, and paranoia. These themes were explored further when Henry Rollins joined the group as lead singer in 1981. Most of the band's material was released on Ginn's independent label, SST Records.

Black Flag was and remains well-respected amongst punk subculture, with their influence primarily in their tireless promotion of a self-controlled DIY punk ethic and aesthetic. They are often regarded as pioneers in the movement of underground do-it-yourself record labels that flourished among the 1980s punk rock bands. Through constant touring throughout the United States and Canada, and occasionally Europe, Black Flag established a dedicated cult following.

Over the course of the 1980s, Black Flag's sound, as well as their notoriety, evolved in ways that both embraced and alienated much of their early audience. As well as being central to the creation of hardcore punk, they were innovators in the first wave of American West Coast punk rock and are considered a key influence on punk subculture in the United States and abroad. Along with being among the earliest punk rock groups to incorporate elements and the influence of heavy metal melodies and rhythm, there were often overt freestyles, free jazz, breakbeat and contemporary classical elements in their sound, especially in Ginn's guitar playing, and the band interspersed records and performances with instrumentals throughout their career. They also played longer, slower, and more complex songs at a time when bands in their milieu performed a raw, fast, three-chord format. As a result, their extensive discography is more stylistically varied than many of their punk rock contemporaries.

Source : http://en.wikipedia.org/wiki/Black_Flag_(band)