Suppose E is a set of entities, L a set of literals (I = E ∪ L is also called individuals), C a set of classes, and R a set of directed relations, a knowledge base K consists of two parts: an ontology O ⊆ C × R × C and a model M ⊆ E × R × (C ∪ E ∪ L).
an ontology specifies classes and relations between classes, and a model consists of facts about individuals.