Who is The Ku Klux Klan from Reconstruction: America's Unfinished Revolution, 1863-1877 and what is their importance?

The Klu Klux Klan (KKK) was a loosely organized group of southern whites who terrorized and murdered blacks and white Republicans across the south. In 1871, Congress passed a law officially outlawing their activities. Few of these men were ever prosecuted.