The Shichibukai, the full title being Royal Shichibukai (王下七武海 Ōka Shichibukai, literally meaning "Royal Seven Warlords of the Sea"), are seven powerful and notorious pirates who have allied themselves with the World Government.
The Shichibukai is an organization of seven pirates turned privateers in the service of the World Government. Although they work for the World Government, the Shichibukai themselves do not care about or respect the Government or even other Shichibukai, and are often considered by the Marines to be no different from any other pirate. On the other hand, they are known as "government dogs" and are despised by other pirates, but are still feared and respected for their infamous reputation and strength before, and even after they become Shichibukai.
The Shichibukai seem to be a mixture of the more chaotic Yonko and the more orderly Marines. The majority of the Shichibukai are composed of New World veterans; those who have met the Yonko or even confronted them. The terms that the Shichibukai are on with each other is best described as very unstable. They have very little (if any) respect for each other and will for the most part not accept help from one another, apparently due to a firm sense of pride.
Benefits of the Shichibukai
In exchange for these services, the Shichibukai are pardoned for their former actions and any existing bounties are rescinded. As the Shichibukai are no longer outlaws, they can make money through legitimate business, as well as bounty hunting. Some, however, are content to simply float around the sea on their ships. The government does not seem to mind, and the Shichibukai are granted a great deal of autonomy in how they operate, at least thus far. Other benefits include access to resources normally out of bounds to normal citizens and pirates, such as the use of Kairoseki equipment.