A Sourcerer is defined as being the eighth son of an eighth son of an eighth son, or a Wizard squared. Whereas Wizards are users of Magic, a Sourcerer is a source of Magic. They can create new spells and change the very fabric of reality to suit their whims.
After the Mage Wars in which much of the Discworld was destroyed by the fallout of Raw Magic from a vast magical battle between the Sourcerers it was decided that it should never again be allowed for Sourcerers to be created. Thus a strict rule was created amongst Wizards that they were to remain celibate; thus ensuring that no Wizard would ever again have eight sons.