The Aes Sedai are a society in the fictional universe of Robert Jordan's The Wheel of Time book series.
In the Age of Legends
In the relatively Utopian society of the Age of Legends, men and women with the ability to channel used the One Power to aid all of mankind—as scientists, healers, philosophers, etc.—constantly developing new innovations and technology to make everyone's life easier. They were therefore known as Aes Sedai, meaning "Servants of All" in the Old Tongue. In spite of the use of the word “servant,” Aes Sedai had considerable status and respect, gained from their use of the One Power. They convened in the Hall of the Servants in the city of Paaren Disen. The greatest Aes Sedai of the Age of Legends was a man named Lews Therin Telamon, also known as The Dragon.
The One Power is divided into male and female halves, known respectively as saidin and saidar. Men who can channel do so using the male half and women use the female half. The greatest works of the Aes Sedai were always performed by groups of men and women channeling together.
Certain Aes Sedai felt that the separation of the two halves of the Power constrained them. Researchers working at the Collam Daan discovered what they thought was a new source of power able to be channeled by both genders. Mierin Sedai (later Lanfear) and Beidomon Sedai bored into it, soon discovering that they had unwittingly enabled the Dark One, formerly imprisoned outside of Time, to influence the world. The power they sensed was in fact the True Power, a dark force akin to the One Power that stems from the Dark One. The former utopia began to slowly decline, until the outbreak of the War of Power, in which many Aes Sedai went over to the Shadow and commanded the armies of the Dark One. The thirteen most powerful 'dark' Aes Sedai were the Forsaken (or the Chosen, to those fighting for the Shadow).
In a desperate move to end the devastating conflict, Lews Therin Telamon and the Hundred Companions led a last-ditch assault on Shayol Ghul, successfully resealing the Bore and trapping the Dark One, as well as (almost) all the Forsaken, outside of the world. However, the Dark One's final counterattack tainted saidin itself, and over time all male Aes Sedai, including Lews Therin, were driven mad. In their madness, they began to channel wildly, causing almost total destruction to everything that hadn't already been wiped out by the war. Some few male Aes Sedai sought shelter in stedding, areas of the world in which channeling was impossible and the taint had no effect, but eventually all male Aes Sedai abandoned the stedding and went mad. By the time the last male Aes Sedai had died, the world map was all but unrecognizable, and the remains of humanity were scattered and destitute. This catastrophic period was called the Breaking of the World.
Among Aes Sedai in the time period of the series, some Aes Sedai, primarily the Red Ajah, believe that male Aes Sedai sheltering in stedding exacerbated the Breaking, making it longer and more damaging than it might have otherwise been. Others, primarily the Blue Ajah, hold that the Breaking might have destroyed the world entirely had all male Aes Sedai gone insane at the same time.
However, female Aes Sedai of the time Foresaw the Rebirth of the Dragon, and undertook several wonders (the Eye of the World, the Stone of Tear, Callandor, etc.) to guide his path and provide him with what he needed.
After the Breaking
On the eastern continent, the remaining female Aes Sedai eventually reorganized themselves on the island of Tar Valon, where, with the assistance of the Ogier, they built the White Tower. The Aes Sedai there became a monastic order, taking in initiates and shaping them into fully-fledged sisters through years of hard training. Their elected leader held the title Amyrlin Seat and became as, or more powerful as any king, queen or legislative body. The Aes Sedai ensured that any man with the ability to channel would be gentled, i.e. severed from the Power. The common people had a tendency to fear the Aes Sedai, knowing that they channeled the same power that had caused the Breaking of the World. As a result, paired with their general aloofness, Aes Sedai have often been the focus of mistrust and even hate, some being convinced that all of them were Darkfriends in service to the Shadow. As a way of making themselves 'safer' and more trustworthy, Aes Sedai began to employ the Oath Rod, taking binding oaths that might make them seem less dangerous.
On the continent of Seanchan, Aes Sedai set themselves up as local governors and rulers rather than establishing a single, united order. During the conquest of Seanchan by the armies of Luthair Paendrag, these Aes Sedai were turned against each other and a device called an a'dam, able to hold women who can channel against their will, was invented. Aes Sedai on the continent were eventually rendered extinct as Luthair Paendrag's successors consolidated control over the entire continent, and Aes Sedai are outlawed in the Seanchan Empire. Instead, all women are tested for the ability to channel at a young age. Those who can channel are made Damane and leashed with a'dam, while those who can learn to channel are made Sul'dam, who control the Damane using a'dam. The Seanchan themselves are unaware that Sul'dam can learn to channel, only that they can control Damane through a'dam, and it is speculated by characters within the series that, if that discovery were to be made, the fabric of Seanchan society might suddenly collapse. Men and women who cannot channel are unable to control Damane using a'dam, while men who can channel or learn to channel feel great pain upon contact with a'dam, and if the contact is long then they die.
Ajahs
In the Age of Legends an ajah was a temporary group of people banded
together for a specific purpose. Today the Ajahs are seven
sub-societies among which all Aes Sedai, except for the Amyrlin, of the
White Tower are divided. These societies each have representatives in the
legislation of the Tower, have personal ideologies, customs and
purposes for joining, their own ruling heads, and all except the White
have networks of spies.. The Ajahs, from largest to smallest, are as
follows: Red, Green, Grey, Brown, Yellow, Blue and White.
There is an eighth Ajah, the Black, which is dedicated to the Dark
One. However its existence was consistently denied by all, and, until
very recently, it was assumed to be myth. Even now many Aes Sedai
within the White Tower do not believe in its existence.