The first organization was formed during the Reconstruction era because they unjustly believed that blacks would greatly have power over whites, despite the fact the freed slaves wanted to be treated as equals, nothing more, nothing less. Much of the KKK members feared that blacks were not just inferior, but that they would intermingle with white people and felt such thing would threaten the white race. They later died out because of the constant intervention by federal troops and the members being tried for violating the civil rights of freed slaves.
The second organization was formed in 1915 due to the racist film "Birth of a Nation" that came out the same year and in mainly in response to the waves of immigration from southern and eastern Europe into the urban centers of the Northeast and Midwest, rather than just being against blacks and Asians. This KKK was extremely anti-Catholic and anti-Jewish, which most of the Southern and Eastern European immigrants were comprised off and were taking jobs that the organization felt should have belonged to the protestant-majority Americans. They felt their faith or unskilled background made these immigrants undesirables in the U.S., feeling they would eliminate American culture and the way of life altogether.
The third and final organization was formed during the Civil Rights Movement, which involved terrorizing civil rights activists, both black and white (many of them from the North), in the South.
Nowadays, they're less than 5,000 members, as the vast majority of mainstream America rejects racism and bigotry as a disgusting reminder how racism torn this country apart.