Ku Klux Klan apparently derived their name from the Greek word kyklos (kuklos), from which comes the English “circle”; “Klan” was added for the sake of alliteration.
The Ku Klux Klan, with its long history of violence, is the most infamous - and oldest - of American hate groups. Although black Americans have typically been the Klan's primary target, it also has attacked Jews, immigrants, gays and lesbians and, until recently, Catholics.
Over the years since it was formed in December 1865, the Klan has typically seen itself as a Christian organization, although in modern times Klan groups are motivated by a variety of theological and political ideologies.
Started during Reconstruction at the end of the Civil War, the Klan quickly mobilized as a vigilante group to intimidate Southern blacks - and any whites who would help them - and to prevent them from enjoying basic civil rights. Outlandish titles (like imperial wizard and exalted cyclops), hooded costumes, violent "night rides," and the notion that the group comprised an "invisible empire" conferred a mystique that only added to the Klan's popularity.
Read more about the history of Ku Klux Klan at the following sites:
The Southern Poverty Law Center
History.com