In the words of Thrull, "Wizards are what all Mages should have been." Wizards are mystically stronger than the Mages, but physically weaker. They get more mana and their spells are more effective, but they also get lower hp gains and are, at best, unimpressive in melee combat. They can also learn some very rare spells by researching them from obscure spellbooks. These spells are not listed on their skill lists.
This is a Prestige Classes for Mages, and they retain the original class's Prime Requisite, in this case, intelligence. Wizards cannot become Sorcerers. They disdain what they see as the Sorcerer's crutch-like dependence on the Dark Powers.