they were southerners after the civil war that disagreed with the new rights the african americans had been given. they started a "secret" (more anonymous) organization to lower the blacks again. they went out dressed fully in white, (even the horses) and terrorized the african people. they would gladly murder, lynch, and destroy any african who opposed them. they were able to get some africans to give up their voting rights, which the KKK wanted.
i hope this helps :)