Akatsuki is a group composed of five S-Rank criminals and missing-nin. They originally had ten, but Orochimaru left and Hidan, Kakuzu, and Deidara have been defeated. Sasori was also defeated, but he was replaced by Tobi, Tobi is later found out to be Madara Uchiha and the true mastermind in the whole Akatsuki organization. Entrance into the group seems to be difficult, due to Sasori's comment, in his battle with Chiyo and Sakura, that "[He] hadn't this much trouble since joining Akatsuki." The team is bent on world domination, and Pain has come up with a three-part plan on obtaining this domination:
The first step is to Obtain large sums of money that will be able to hold them through the other two steps of the process. The second step is to solidify a reputation, this step will be aided by the Bijuu. Akatsuki, after capturing all of them, will unleash them on different villages and then seal them again. They will also start taking jobs at a cheaper rate than any village so that everyone will want to hire them, financially crippling the major villages. The third plan is to take over the other villages when their countries are in a state of economic emergency.
However it is revealed that Yahiko (before he became the deva pain) and Madara Uchiha (who is also Tobi) formed Akatsuki and Madara plans to fuse the tailed beasts into their original form the jūbi (Ten-Tails).
But even this was all part of a larger plan. The reason they want to unleash the Ten-Tailed beast is to achieve the eye of the Moon plan (Tsuki No Me Keikaku) and put the world under an eternal genjutsu known as the Infinite Tsukuyomi.
After Kabuto Yakushi revives the real Madara Uchiha via reanimation, Tobi is revealed to be in fact Obito Uchiha, one of Kakashi's old comrades who was thought to be killed in action during the Third Great Ninja War, and has been posing as Madara ever since.
At this time the Akatsuki was reduced to just Obito and Zetsu after the death of Kisame Hoshigaki and the betrayal of Nagato and Konan. They then allied with Kabuto to begin the Fourth Great Ninja War.
They all dress in the same clothing but some a little different. It is a long black coat with red clouds on it, they also have their nails and toenails polished, and when they're on a mission they often wear decorated straw hats. The team still wear the head band from their former village, but a scratch through the symbol that symbolizes their severance from that village; although some members' headbands aren't visible.
Every single Akatsuki member wears a ring, the total rings are ten and every last one of them is unique. Tobi wanted to be a member because he found Sasori's ring, Zetsu says its not going to be easy but he does indeed join and becomes part of the Akatsuki. When Deidara loses both of his arms, he is more concerned in finding the ring instead of finding a new arm. When Orochimaru leaves the Akatsuki, he took the ring with him and didn't give it back. Each ring holds a symbol of their nickname in Japanese characters. No ring can be on the same finger as someone else, each have their own ring to put on one finger.
The right thumb is Pein and his kanji is "Zero" which is on his ring in Japanese characters. The right index finger is Deidara and his kanji is the "Azure Dragon". The right middle finger is Konan and her kanji is called "White Tiger". The right ring finger is Itachi Uchiha and his kanji is "Vermilion Bird". The right pinkie finger is Zetsu and his kanji is "Black Tortoise". The left pinkie finger is formerly Orochimaru's and his kanji is "Void". The left ring finger is Kisame Hoshigaki and his kanji is "South Star". The left middle finger is Kakuzu and his kanji is the "North Star". Hidan's kanji is "Three Levels" and his is the left index finger. The left thumb worn by Tobi formerly Sasori, is called Virgin.