Not much is known about the origin of the Wither Skeleton, other than that they came through the dark portal along with other forces of Gorgath's armies. They have a shyness to sunlight and prefer the cover of trees. However when engaged in combat they have no issues with leaving the darkness to catch a humans blood.
A Wither Skeleton is primarily armed with a bow, with a fire particle appearing out of it. However sometimes also use Swords and Axes.
A common tactic for fighting a Wither is to take advantage of its height, it is much taller than the common Andalucian and stands three blocks (9M) high.
Also when in the Dark Oak Forest you can take advantage of tree cover, using it to avoid archer attacks and you can wound, Retreat and finish them off.