The Ku Klux Klan (KKK) is an organization in the United States dedicated to opposing civil rights for Blacks, Jews, and other disadvantaged groups. In certain times and places it has also opposed Catholicism, and 'left' groups such as the IWW, and the gay rights movement.
The name Ku Klux Klan comes from kuklos, the Greek word for circle. A persistent myth has existed in the United States which alleges that the name comes from the sound of a rifle bullet being chambered, but this is false.
The Klan is associated with acts of terrorism such as lynchings and burning of homes, and a consciously mystical and grandiose aesthetic involving wearing white robes and hoods and burning crosses.
The Ku Klux Klan was first established after the end of the American Civil War, to oppose the extension of voting rights to Blacks, and other measures to end segregation, introduced as part of the Reconstruction. But as federal control of the ex-Confederate states was withdrawn, the local white population re-established their power and with it segregation laws. The Klan then faded from existence, its purpose served.
The Ku Klux Klan was re-established during the Great Depression, a feat which arguably would not have been possible without D. W. Griffith's skillful propaganda film The Birth of a Nation, based on the play The Clansmen and the book The Leopard's Spots, both by Thomas Dixon. Many poor whites were drawn to the idea that their economic woes were caused by Blacks, or by Jewish bankers, or by other such groups. The Ku Klux Klan rose to great prominence and spread from the South into the Midwest and Northern states, and at this time counted many politicians among its members. However, with the recovery from the Great Depression and the onset of World War II, the Klan again faded; it has never recaptured the number of members it held then.
The third Ku Klux Klan was established to counter the Civil Rights movement of the 1960s. This is the Klan that is still seen today, though as American society has become more racially tolerant the Klan has once more shrunk dramatically and fractured. The major factions currently include the Imperial Klans of America, the American Knights of the Ku Klux Klan, and Knights of the White Kamelia.
See also Jim Crow laws
The Southern Poverty Law Center -- http://www.splcenter.org/splc.html