Description logics (DL) are logics serving primarily for formal description of concepts and roles (relations). These logics were created from the attempts to formalize semantic networks and frame based systems. Semantically they are found on predicate logic, but their language is formed so that it would be enough for practical modeling purposes and also so that the logic would have good computational properties such as decidability. The focus of research in DLs is how the various DL constructs are usable for real world applications and what is the impact of them against the complexity of reasoning.
Knowledge representation system based on DLs consists of two components - TBox and ABox. The TBox describes terminology, i.e., the ontology in the form of concepts and roles definitions, while the ABox contains assertions about individuals using the terms from the ontology. Concepts describe sets of individuals, roles describe relations between individuals.