CP9 is a "secret" organization, formed of people able to use rokushiki, six powerful skills only known to its members. CP9 also has special privileges not available to normal Cipher Pol squadrons, like the ability to kill any person perceived as a threat to world stability. More recently, CP9 was gifted, under the authority of Admiral Aokiji, the ability to initiate a "Buster Call", a powerful strike force of ten warships which hunt down and destroy any particular criminal group unfortunate enough to be targeted. There are currently nine members of CP9.
Four of CP9's agents have spent the last five years in the One Piece' timeline undercover as shipwrights in the Water 7 based Galley-La Company. The organization seeks the plans to a powerful, ancient warship, called Pluton (the same ship mentioned in Alabasta's Poneglyph that was Sir Crocodile's main objective), in order to bring an end to the "Great Pirate Age," sparked by the death of Gol D. Roger. Due to their extreme power, it seems that they all have extremely arrogant and pompous attitudes.
The CP9 are based in Enies Lobbys' Tower of Justice, where they act as the final guardians of the Gate of Justice. It has been said that Enies Lobbys reputation of being impossible to conquer is because of the generations of CP9, and their strength.