A FactCreater is a helper class specifically for the Business Rules Composer that give you the ability to Assert, Short Term, non TypedXmlDocument facts, such as instances of .Net Classes, when you do Test Policy in the Composer. It is not used outside of the Composer.
A FactRetriever is used to Assert Facts, called Long Term, behind the scene buy the engine. It is used whenever the Policy is executed, including by the Composer.
A FactCreator is effectively the Composer's equivalent of the CallRules Shape in an Orchestration, where you specify the Short Term facts.