The Dark One is the Randland equivalent of our Satan.  I am sure that is where Robert Jordan got his name, Shai'tan.  The only difference is that, in our world, Satan turned evil and, in Randland, Shai'tan was always evil.  Shai'tan has no physical form or gender but is generally referred to as a man.

    He is a man of many names, including: Father of Lies, Sightblinder, Lord of the Grave, Shepherd of the Night, Heartsbane, Soulsbane, Heartfang, Old Grim,  Grassburner, Leafblighter and, though almost never spoken, Shai'tan.

    Through all eternity, Shai'tan has wanted to break free of his prison and come into the world physically and take on a human form.  If he ever succeeds in this, he will attempt to remake the world according to his likeness.  One Darkfriend-turned-to-the-Light, said that apparently Shai'tan is emencely displeased with its look at the present time.  Many theologians have different views on Shai'tan but one very common thought is that the Dark One is, in fact, part of the Creator.