What’s the difference between void ab initio and voidable?

A shelter manager for the Delaware SPCA claimed that the board’s decision to fire him from his job was void ab initio and therefore ineffective because the board was comprised of less than 20 individuals as required by a special state law.  A federal District Court in Delaware has dismissed the count of the complaint for wrongful termination, but granted leave for the manager to plead facts to show that the termination was voidable.