A sorcerer was a magic-user that could not be harmed directly by a wizard's magic. Their Magic and wizard's magic were different - like oil and water. Likewise, a sorcerer could not directly harm a wizard with his magic.
On someone without the gift however, the magic of a sorcerer would work just as effectively, though in a different way to a wizard's.
Sorcerers were rare - about as rare as War wizards. Brother Narev was the first to be born in nearly three thousand years. He was thought to be a wizard by his followers, but was really quite different entirely. He sought to kill Richard Rahl with his magic, not knowing who he was and that he had the gift. This plan, in return, backfired, and in Narev's moment of surprise, Richard killed him.