Given the numerous knowledge representation models (KR-schemes) that have been proposed, it would be desirable to have a formal, unifying model for the description of a KR-scheme, as well as a general method of comparing KR-schemes in terms of expressive power. This work attempts to fill this gap, by proposing an elegant, yet very general, model of describing KR-schemes. This formalization is used to describe any knowledge representation model, including databases, logic-based schemes, semantic networks etc. It is also applied to introduce a general comparison method for KR-schemes and a formal definition of the reduction of one scheme to another. Using this model, we can reason -schemes in an abstract manner and to determine whether a certain reduction is possible or not.