The One Power is divided into male and female halves, saidin and saidar respectively, that work both with and against each other to drive the Wheel of Time. The True Source cannot be consumed by channellers, as a river cannot be consumed by a waterwheel. Each half of the One Power is able to be used only by people of the associated sex (the only known exception is the Forsaken Aran'gar, a male channeler who was re-incarnated into a female body but still uses saidin), and men and women have very different experiences in channeling.
Not everyone has the ability to "channel" the One Power. Most men and women cannot reach it at all; of those who can, most must be taught to access it to varying extents, and can go their entire lives without touching (or even knowing they can touch) the Source. Generally they find out when a member of the series' only formally-organized group of channelers, the Aes Sedai, visit their town on a recruiting trip. Aes Sedai visitors also search for the few channelers who are born with "the spark," an innate gift that will inevitably lead them to touch the Source. If left untrained, these gifted channelers often die without ever gaining control over their innate ability; the Aes Sedai estimate the mortality rate at 75% or more. Those who do survive, called "wilders" by the Aes Sedai, do not realize that they have channeled, only that "things happen" if they want them to. To aid in this self-deception, they are frequently saddled with a "block," an instinctive self-preservation behavior that keeps them from channeling in all but exceptional circumstances. Known blocks include only being able to channel while angry, in the presence of men, with one's eyes closed, and so on. These blocks can be eventually overcome, but only after significant and sometimes drastic counseling. The trainee, Theodrin Dabei, who could only channel in the presence of men, for instance, was broken of her block when the handsome young stableboy who had been attending her training sessions revealed that "he" was actually the handsome young stableboy's twin sister.
Channelers must first tap into the One Power before being able to channel it, and can "hold" the Power within themselves without using it; the feeling of accessing the Source is described as being "more alive than ever before," a powerful feeling of joy for women, and a feeling of "holding a river of frozen fire and burning ice" for men, as well as a significant increase of sensory acuity. The "sweet" and addictive nature of the Power is problematic in itself, as drawing too much of it can kill the channeler, render them brain-dead, or—if they are lucky, relatively speaking—only "burn out" the ability to channel permanently. Channelers can also be temporarily cut off from touching the Source using shields woven of Spirit, and or permanently cut off from it by outsiders; modern Aes Sedai call it "stilling" when done to women and "gentling" when done to men, while historically it was called "severing" for either sex. Regardless of the circumstances, those who lose the ability permanently also tend to lose the will to live, and are often dead within a year. Very recently, Aes Sedai Nynaeve al'Meara and Asha'man Damer Flinn have (independently) discovered how to heal severing, but the process only works completely when the healer is of the opposite sex to the healed; when Nynaeve healed fellow Aes Sedai Siuan Sanche and Leane Sharif from their stillings, they were left with significantly reduced strength.
Channelers are not born equal: some are stronger in the Power and can handle larger quantities of it than others. Men and women, furthermore, are different in many regards when it comes to the One Power, as covered below.
The One Power is "woven" (or "spun" in the terminology of those from the Age of Legends) in "flows" or "threads" (or "webs") consisting of one of five elements: Earth, Water, Air, Fire and Spirit. Men are generally stronger with Fire and Earth, while women are generally stronger with Air and Water; strength in Spirit is typically equal, and equally rare. Male channelers, in general, are capable of holding more of the Power than female ones, while women generally are more dextrous in weaving the Power. Also, male channelers cannot link to form a "Circle" of multiple channelers working together without a woman's aid; only women can initiate a Circle, although there cannot be more men than women in a circle and no more than thirteen women may be joined without including any men. These differences allow for rough parity between male and female channeling, and it is known that the greatest works of the Age of Legends (a golden age that ended some 3,000 years before the start of the novels) were performed by men and women channeling in cooperation.
Female channelers can tell when other women are touching the Source: a white glowing aura appears around their body, though only visible to those trained to touch the One Power. It is also relatively easy for them to gauge each other's strength in the Power. Men, on the other hand, can only sense intuitively when another man is channeling (and roughly how much of the power they are weilding), and both must weave saidin in each other's presence for a length of time (half an hour or more) before being able to even guess at the other's strength. Male channelers can sense a woman holding saidar as goose bumps on their skin; women have no innate method of sensing male channeling, though certain ter'angreal and weaves have been crafted which do the job for them.
Most vividly, male and female channelers experience the Power differently: a woman would describe it as a gentle force that she must submerge herself in, as trying to exert control over it would lead to being instantly consumed; a man, on the other hand, faces a cataclysmic torrent requiring perfect control, immense willpower and ceaseless vigilance. In short, women "embrace" saidar, surrendering to it and influencing it from within, while men "seize" saidin, manipulating it by force. This essential difference in the "feeling" of the Power means that a woman cannot teach a man to channel, and vice-versa (though it has been tried—many, many times), especially since many weaves which work for one sex do not work the same way (or at all) for the other. As an example, men can use flows of Fire to transfer the heat of a candle flame to another location (for instance, the stones of a fireplace), but women who try that have been grievously injured, some even bursting into flame—just from a candle. (Women use a thread of Water or Air to snuff a candle.) Male and females also grow in the One Power differently; women gradually increase in strength, whereas men gain strength in sudden leaps.
Terminology differs between the sexes, as well. Women, as stated before, "embrace" saidar while males "seize" saidin. Males "weild" their half of the One Power as females "guide" it. Women are "stilled" when they are cut off from the True Source while men a re "gentled"; a gender neutral term was used in the Age of Legends, "severing". "Burning out" (an accidental self-severing from the One Power occurring when one mis-handles or tries to use too much of the One Power) is gender neutral as well.
Some other differences include:
In the current age, the Dark One's taint on saidin causes any male with "the spark," whether or not he learns to channel safely, to inevitably go mad and succumb to a wasting sickness which causes the sufferer to rot alive (these curses may come in either order, or concurrently, at varying speeds for each male channeler). The taint came into being at the end of the Age of Legends, when Lews Therin Telamon and the Hundred Companions (actually 113 male channelers) re-sealed the Dark One into his prison, but not before the Dark One caused saidin to become tainted, driving all of those attempting to seal him immediately and irrevocably insane (other male Aes Sedai took longer for the taint to affect them). The ensuing chaos and destruction caused The Breaking of the World or the Time of Madness. Men who can channel, in short, are a threat. With this in mind, members of the Aes Sedai formed a faction, the Red Ajah, dedicated to the hunting down and gentling of male channelers before they can cause (much) trouble. As with all severed channelers, these men rarely live for long afterwards, and few have children. Coincidentally, by the time of the novels, fewer and fewer people of either gender are being born with any channeling ability. The logic-minded White Ajah have suggested that there is a link between this and the thousand-plus years of gentling, comparing it to the breeding-out of distasteful traits in domesticated animals. Despite this, increasingly powerful channelers have been discovered throughout the series, though this could be due to the effects of Ta'veren.
At the conclusion of Winter's Heart, the ninth book in the series, saidin has purportedly been cleansed through the use of two of the most powerful sa'angreal ever made, called the Choedan Kal. They were used by Nynaeve al'Meara and Rand al'Thor in a linked Circle. The complex process destroyed the ruined city of Shadar Logoth and was "felt" by most, if not all, channelers of both genders in the Westlands. However, some have confessed doubt over the effectiveness of the cleansing, due to three thousand years of fear. The question was not answered definitively until the eleventh book, Knife of Dreams, in which both Rand al'Thor and various Asha'man confirm that saidin has been cleansed completely of the taint. However, any taint absorbed by a male channeler before saidin was cleansed was not removed from that channeler; men who had gone half- or entirely mad due to the taint were not returned to sanity, though they will also not suffer further deterioration.
Many other cultures have their own groups of channelers, such as the Sea Folk's Windfinders, the Aiel Wise Ones, and a network of expelled or disgraced Tower trainees, the Kin, who operate under the Tower's nose; however, these groups keep themselves secret because of the open Aes Sedai ambition to control every living female channeler. Relatedly, some Aes Sedai will disparage any female channeler who is not Tower-affiliated and Tower-trained as a "wilder," even if she has received training from her own culture's institutions.