The Shichibukai are an organization of seven former pirates turned privateers in the service of the World Government. The Shichibukai do not care about or respect the Government (excluding Bartholomew Kuma) or even other Shichibukai.
The Shichibukai seem to be a mixture of the more chaotic Yonkou and the more orderly Marines. There is an enormous range in the bounties between the members, with Donquixote Doflamingo, who has the highest known former bounty of the Shichibukai (or any other known person) at Image:Bsymbol.gif 340,000,000, Hancock with a comparatively low bounty of Image:Bsymbol.gif 80,000,000 and Blackbeard, with no former bounty at all.
The terms that the Shichibukai are on with each other is best described as very unstable. They practically have very little (if any) respect for each other and will for the most part not accept help from one another, apparently from a firm sense of pride. During the battle meeting for the Shichibukai where only two members are absent (Hancock and Jimbei) it is even stated that the very notion of them working together as a team is entirely unthinkable.
During Boa Hancock's visit at Impel Down's sixth level of detenction, Jinbei stated that he didn't know much about her, possibly stating that only some of the Shichibukai are familiar with each other or even have personally met all the other members.
Only four of the Shichibukai currently remain loyal to the World Government.
Becoming a Shichibukai
A pirate may choose to join the Shichibukai if a space becomes open (which means that at least one Shichibukai must leave or be kicked from the group, the former being Jinbei's case and the latter being Crocodile's case). However, in order for one to become a Shichibukai once this position opens up, one must be able to show their strength against other pirates while choosing to pledge loyalty to the World Government (this oddly promotes pirates to gain infamy to become a Shichibukai). Once they become renowned, the position may be offered to them, if no other pirate has accepted the position or proven to be more of an asset to the World Government. As expected, most members were already notorious pirates with high bounties, but even relatively unknowns can be admitted if they accomplish some feat to prove their strength, such as defeating someone with a world-wide known strength like when Blackbeard defeated Ace.
Duties of the ShichibukaiThey do not offer open opposition to the rule of the World Government, but are not guaranteed to follow its orders either. They are only allowed to attack other pirates and must also pay the Government a tenth of their loot. Each member of the Shichibukai must also have strong intimidation skills and a fearsome representation to put off others becoming a pirate and make them fear them. In addition, given the dialogue between Bartholomew Kuma and the Gorousei, it can be assumed that a member of the Shichibukai can not be defeated, or have widespread knowledge of a said defeat (or at least by rookies who are gaining influence such as the case of Luffy who has been able to defeat Gecko Moria).
Some of the members do various tasks for the government, such as taking over new islands for the World Government (described as "liberating") and guarding nations, but their ultimate purpose is to keep other pirates in check. Most importantly, the Shichibukai serve to help keep the balance of The Three Great Powers.
Theoretically, the Shichibukai are allies of the government, and in war time would fight alongside the Marines. Still, many Marine officers (for example Commodore Smoker, Admiral Kizaru and Fleet Admiral Sengoku) dislike them and think of the Shichibukai just as pirates. Several high-ranking Marines expressed surprise that even two of the Shichibukai actually bothered to show up when summoned for the meeting about Crocodile's removal, and they were very shocked when a third arrived. Tellingly, of the others besides Kuma that showed up, Doflamingo came out of sheer boredom (passing the time before the meeting by nearly forcing two marines to duel each other), and Mihawk came only to hear about the pirates being discussed in the meeting. When they are not under orders of the World Government, they have no commitment to aid the efforts of the Marines. Most of the Shichibukai appear to show general apathy towards the World Government and in some cases even hostility (such as Boa Hancock who openly attacked both Marines and pirates during the battle with Whitebeard).
AffiliationsAside with the World Government, the Shichibukai are (or were) affiliated with another group(s).
* Juracule Mihawk: Former rival of Red-Haired Shanks, remains in friendly contact with him; He is interested in Roronoa Zoro's swordsmanship and sees him as a future rival and potential successor for the title that he's actually carrying: World's greatest swordsman.
* Jinbei: Second Captain of the Sunny Pirates (crew disbanded and split); Has befriended Monkey D. Luffy; resigned his Shichibukai status upon his arrival at Marineford.
* Crocodile: Former president of Baroque Works (disbanded); Arrested, stripped of his title and reinstated as a pirate; Temporary ally with Monkey D. Luffy in getting out of Impel Down (Allegiance status in question upon arriving in Marineford)
* Donquixote Doflamingo: Former leader of the Bellamy Pirates (abandoned); Former owner of the Human Auctioning House (donated it to Disco).
* Bartholomew Kuma: Part of Pacifista program created by Vegapunk; Some form of past relationship with the Revolutionaries.
* Gecko Moria: Former Captain of an unnamed crew (presumably massacred by Kaidou); Former leader of Thriller Bark (presumably destroyed) and former leader of the Mysterious Four (partly disbanded because Perona is no longer with him).
* Boa Hancock: Former slave of Tenryuubito (freed by Fisher Tiger); Captain of the Kuja Pirates; Empress of Amazon Lily; She is in love with Monkey D. Luffy and defected from the Shichibukai to protect Luffy.
* Marshall D. Teach: Former subordinate of the Whitebeard Pirates second division (defected after killing Thatch); Captain of the Blackbeard Pirates.
Frictions Against the World GovernmentDespite being allies of the World Government, most of the behavior exhibited by the Shichibukai have caused much conflict against their employers.
* Jinbei: He refused to fight on behalf of the World Government against Whitebeard, he forfeited his position as a Shichibukai member and was imprisoned in Impel Down. Currently he's escaped from Impel Down and is waging war against Marineford in order to prevent Ace's death. After landing in the war, Jinbei told Sengoku that he has quit being a Shichibukai.
* Sir Crocodile: He abused his title and privileges, was stripped of his status, arrested and then imprisoned in Impel Down Has invaded Marineford with Monkey D. Luffy and Jinbei.
* Bartholomew Kuma: He assisted the Straw Hat Pirates by allowing them to escape from death in two almost consecutive situations: During the aftermath of Thriller Bark and during the battle against Admiral Kizaru, Sentoumaru, and a Pacifista.
* Boa Hancock: She initially refused to live up to her title as Shichibukai and fight Whitebeard, and she accepted to fight Whitebeard only because this allowed Monkey D. Luffy to rescue important prisoner Portgas D. Ace (latter action yet unknown to World Government). Even though she agreed to fight against Whitebeard, she has made it clear that the Marines are not her allies by attacking without any distinction both pirates and Marines during the war. She has openly assisted Luffy by striking Commodore Smoker and even declared Luffy as her beloved, effectively defecting from the Shichibukai.
* Marshall D. Teach: He stole a Marine ship with his crew, using it to break into Impel Down for currently unknown reasons, attacking the prison guards in the process. He recruited Shiryuu into his crew (latter action yet unknown to World Government).
Apparently, the only members who did not cause any major conflicts against the World Government are:
* Juracule Mihawk: He did get acquainted with the Straw Hat pirates prior to the Marines becoming aware of them and appear to be a friend of Yonkou member Shanks.
* Donquixote Doflamingo: His former ownership of the Human Auction House may had some impact on the justice system, but apparently not enough to lose his position.
* Gecko Moria: His abduction of civilians and Marines was troublesome and his defeat against Monkey D. Luffy almost caused a global uproar if it hadn't been covered up.
StrengthThe Shichibukai are a group of seven of the strongest pirates in the world and are one of the 3 Great Powers, the others being the Yonkou and Marine Headquarters.
In terms of power, the exact relationship between the Shichibukai, Marine Headquarters and Yonkou is unknown. However, considering that these organizations are stated to be a balance and the fact that the loss of Crocodile threatened said balance, the most reasonable conclusion is that each organization is roughly equivalent in strength. At least two of the Shichibukai (Moria and Mihawk) are said to have been rivals of Yonkou members, Kaidou and Shanks respectively. Bartholomew Kuma has also allowed himself to become part of the Pacifista project. The strength of at least one Shichibukai, Boa Hancock, has been acknowledged by Fleet Admiral Sengoku himself.  This, in addition to the fact that losing a single member of the Shichibukai can throw off the balance of the world powers and cause distress to the World Government, indicates the extreme power each individual has. The strength of a Shichibukai is so great, that should they be imprisoned for any reason, they would be sentenced to Level 6 of Impel Down, the lowest and most secure level of the prison (as both Crocodile and Jinbei were sentenced).
Because of their deal with the World Government, the Shichibukai are considered "Government Dogs" by other pirates. At the same time, they are also feared by them because all of the Shichibukai are insanely strong, each capable of destroying an average pirate crew on their own with no effort as seen with Mihawk and Crocodile. They have such a fierce reputation that even other powerful and famous pirates are unnerved by them as seen with the captain of the Kid Pirates, Eustass Kid, actually seemed heavily disturbed when he witnessed the arrival of a Pacifista cyborg (mistaking it for the real Kuma) on the Sabaody Archipelago, close to where his crew were, despite the fact that his bounty was higher than Kuma's former bounty.
Each member seems to have some certain ability or power, the majority of which are Devil Fruit. These abilities can often times vary depending on the Shichibukai's theme. Some of their abilities appear to contradict each other: for example, Blackbeard's power involves drawing people to him, while Kuma's involves rejecting people away from him; and Doflamingo's power allows him to control the motion of other people to his will, while Hancock immobilizes them by turning them into stone. In the case of Crocodile and Jinbei, this opposition also extends to the environment where their abilities reach their full zenith (i.e. being a Fishman, Jinbei's fighting abilities are at their peak in the deep and open sea, while Crocodile, having consumed the Suna Suna no Mi, favors the dry desert).
Abilities and Powers* Juracule Mihawk: Kokutou Yoru; Pendant cross dagger; World's greatest swordsmanship.
* Jinbei: Fishman Super-Human Strength; Fishman Karate master; Ability to talk to fishes.
* Crocodile: Suna Suna no Mi, Logia; Gold-alloy hook with scorpion poison and a hidden blade.
* Donquixote Doflamingo: Unknown power that allows him to manipulate people like puppets and cut with invisible strings.
* Bartholomew Kuma: Nikyu Nikyu no Mi, Paramecia; Pacifista armor and laser.
* Gecko Moria: Kage Kage no Mi, Paramecia; Zombie army; Giant pair of scissors.
* Boa Hancock: Mero Mero no Mi, Paramecia; Haoushoku Haki.
* Marshall D. Teach: Yami Yami no Mi, Logia; Super-Human Strength.
Former BountiesThese bounties were the last given to the members before they joined the Shichibukai, so they may not represent the threat levels they would possess if they were still enemies of the World Government.
* Juracule Mihawk: Unknown.
* Donquixote Doflamingo: Image:Bsymbol.gif 340,000,000
* Gecko Moria: Image:Bsymbol.gif 320,000,000
* Bartholomew Kuma: Image:Bsymbol.gif 296,000,000
* Jinbei: Image:Bsymbol.gif 250,000,000
* Crocodile: Image:Bsymbol.gif 81,000,000
* Boa Hancock: Image:Bsymbol.gif 80,000,000