In The Wheel of Time, the male half of the One Power, drawn from one and the same True Source. Channeling basically means that you do magic, and it is divided by genders, so that humans of one gender can not touch the power of other side. Only few humans have ability in channeling, and more die learning it. OP is a powerful weapon, powerful enough to break the world, which is why it is regulated by Aes Sedai (organization of those able to wield OP) who take oaths not to use it as weapon.

Wielding of saidin is described as fighting for the control: channeler "seizes" the saidin and forces it to his own will, as opposed to saidar style of "surrendering" to the power and guiding it. This reflects the ideas Robert Jordan has about how men and women impose their will upon others.

Saidin, especially, is dangerous to wield as Dark One has lately tainted saidin, so that anyone wielding it will collect taint. The more taint you have, more insane you go, eventually killing yourself (and everyone around you).